(Patterson Clark/The Washington Post)

The United States and five other world powers reached a deal with Iran over its controversial nuclear program Sunday. The agreement sets stringent limits on Iran's nuclear activities; in exchange, the country will get about $6 billion in unfrozen foreign assets and relief from sanctions. Some people think it's a good deal, some think it's bad deal, but everyone agrees it's a big deal.

For people who have not been following every twist and turn of the Iranian nuclear dispute, which is just about everyone, this story can get overwhelming. There are the decades of history leading up to it, deeply contentious diplomacy by several countries that want very different things and, of course, the nuclear science of what Iran can and cannot actually build. It can be a lot to keep straight, not least because some of the most important details are disputed.

Here, then, are the most basic answers to your most basic questions. First, a disclaimer: This is not an exhaustive or definitive account of this very complicated story, just some background, written so that anyone can understand it.

1. What is Iran's nuclear program?

This question is the entire conflict. Iran says its nuclear activities are peaceful, but a lot of countries worry that they're cover for a nuclear weapons program. The dispute, on the most basic level, is over what sort of nuclear program Iran gets to have -- if any at all -- and what happens if it defies the world's demands.

Iran has been developing nuclear fuel and technology for years, which it says is just for power plants and scientific research. They've got a few big facilities, some of which are out in the open and some of which are hidden away in underground bunkers. The program, and this is where it gets controversial, includes some stuff that would be awfully useful if Iran wanted to go a step beyond a peaceful program and develop a nuclear bomb.

2. So is Iran building a nuclear bomb or not?

It's not clear. The United States and several other countries believe that Iran is trying to develop the technology and fissile material necessary to build a nuclear weapon. There's an important distinction here: Western intelligence agencies have not concluded that Iran has decided to definitely build a bomb. Rather, they've reported lots of signs -- secret facilities, weapons-related research programs -- that suggest that Iran is trying to develop the technology and materials necessary to build a nuclear bomb very quickly. This is called "breakout capability," as in Iran would have the ability to quickly "break out" into a full-fledged nuclear weapons state.

The United Nations' nuclear watchdog hasn't definitively concluded that Iran is doing this, but it has reported some very worrying signs and says it can't state confidently that the program is peaceful. Iran has also dodged inspections and built secret facilities, which is not exactly reassuring anybody.

The world is so worried about Iran's nuclear intentions that, starting in 2006, even China and Russia joined with the rest of the United Nations Security Council -- a small, powerful body of world powers -- in ordering Iran to "suspend all enrichment-related and reprocessing activities, including research and development." Iran has not complied, insisting that its program is a point of national prestige and independence. It's been punished severely with economic sanctions, including on its vast oil and gas industry.

The unresolved conflict over Iran's nuclear program has left the once-wealthy country increasingly impoverished, harming especially its large middle class. It's also bad for European economies, which are losing out on all the business they'd do with this large, resource-rich country. It's terrified Iran's neighbors, particularly Israel and Saudi Arabia, who are worried what Iran would do with a nuclear weapon. And it's been a major part -- but far from the only part -- of Iran's long-standing tension with the West, especially the United States, in which war is a remote but real possibility.

3. Wow, Iran's nuclear program is causing some major problems, especially for them. Why do they insist on it?

There are two very different explanations for why Iran is so adamant about its nuclear program. There's probably truth to both.

The first explanation is that the nuclear program has huge symbolic importance for Iran. You have to understand that Iran's national pride runs deep, and with good reason: It has been an active center of cultural, scientific, religious and political thought for many centuries. It's also still upset, again with reason, about decades of Western interference during the 19th and 20th centuries. The nuclear program is a way in which Iran affirms, to itself and to the world, that it is an advanced and sovereign nation. It's also a way of defying what it sees as continued Western efforts to control, exploit or weaken Iran.

That's what Iranian leaders mean when they talk about the nuclear program as a point of "national dignity," as they often do.

The second explanation is much more straightforward, but it's not one that Iranian leaders acknowledge: defense. If Iran is pursuing some sort of nuclear weapons capability, then logically this would be at least partially for defensive reasons. Most analysts believe that Iran would want a nuclear weapon to deter perceived foreign threats. Consider Tehran's view for a moment: Israeli and American leaders have been talking for years about bombing Iran or invading it outright. The Bush administration named Iran part of its "axis of evil," alongside Iraq, which it invaded months later.

Iranian leaders appear to sincerely believe that the United States is bent on their government's destruction. For example, the United States helped Iraqi leader Saddam Hussein in his brutal, years-long war against Iran, in which he killed thousands of Iranians, including with chemical weapons. You hear Iranians frequently mention Iran Air flight 655, a civilian airliner that the U.S. military accidentally shot down in 1988, killing 290 civilians. In Iran, this is still frequently viewed as deliberate. Imagine you're an Iranian leader seeing all this. You might want a nuclear deterrent.

4. Lots of countries have nuclear programs. Why would it be so bad if Iran did, too?

Scroll back up and reread the second answer. Maybe, 10 years ago, Iran could have convinced the world that it should be trusted with a peaceful nuclear program. But it has burned through that trust so completely that even China, which hates the idea of Western-imposed restrictions on any country, voted to forbid Iran from having any uranium-enrichment program at all.

Still, Iran has a point when it says that the world has unfairly restricted it from even peaceful nuclear energy. The 1968 nuclear Non-Proliferation Treaty, which almost every country in the world follows and which basically sets the global nuclear rules, says, "Nothing in this Treaty shall be interpreted as affecting the inalienable right of all the Parties to the Treaty to develop research, production and use of nuclear energy for peaceful purposes."

When the U.N. Security Council told Iran that it was not allowed to have any nuclear program at all, wasn't it violating Iran's rights? You can see how Iranians might see this as a continuation of decades of foreign interference, making it all the more important that they resist.

But here's the thing: Iran has engendered such deep suspicion that its peaceful program is cover for a weapons program that the two are no longer really separable. The world today cares a lot more about preventing Iran from developing a nuclear weapon than honoring its right to peaceful nuclear energy. The burden is now on Iran to prove that it's not up to anything.

5. This is all really complicated. Can we take a music break?

Good idea, and a nice excuse to at least nod to the fact that Iran is much, much more than just its nuclear program. It has a long musical tradition that extends right up to today. Here's a song by the great contemporary Iranian group Niyaz, which combines traditional Persian music with modern electronic and rock.

Niyaz sometimes tours in the United States and has a new-ish E.P., but my favorite of theirs is the 2008 album "Nine Heavens." Okay, now back to the nuclear stuff.

6. Can you just explain, in really simple terms, the science of how the Iranian program works?

You need two things to build a nuclear weapon: fissile material and a warhead. It doesn't appear that Iran has either, but there are pretty strong signs they might be working toward both.

The warhead is straightforward: The United Nations nuclear watchdog agency announced in a 2010 report that it had found evidence that Iran's military was working on the technology to develop a nuclear warhead. Western intelligence agencies have said they'd found earlier signs of this as well.

The more important and controversial part of this is the nuclear fissile material. You need one of two sources to build a nuclear bomb: highly enriched uranium or plutonium. There are signs that Iran could be working on both.

First, the uranium. Uranium is a silver-colored metal that occurs in nature. Straight out of the ground, only about 0.7 percent is made up of the isotope uranium-235, which is what makes it usable for nuclear processes. To do anything with uranium, you need to "enrich" it using centrifuges, special devices that spin it at extremely high speeds and pull out that uranium-235 isotope. Once you've spun the uranium to the point where 3.5 percent of it is uranium-235, we say it is "3.5 percent enriched" and it can be used as fuel for nuclear power plants. If you want to use it for a nuclear bomb, you have to get it all the way to 90 percent enriched, which requires special technology and is a lot tougher to do.

Okay, now you understand the basics of uranium enrichment -- congrats! Here's what it has to do with Iran. The country has a stockpile of uranium that it's enriched to 3.5 percent. That violates U.N. demands that Iran stop all enrichment activities, but on its own 3.5 percent uranium is not a big deal. The issue is that Iran has vast, underground enrichment facilities with thousands of specialized centrifuges, which could theoretically be used to move toward weapons-grade uranium. More to the point, Iran has a stockpile of 20 percent enriched uranium. This is not weapons grade. But it is relatively easy to pop it up to 90 percent. This is why a major part of the new deal with Iran requires it to "neutralize" its stockpile of 20 percent uranium.

Now for the plutonium. Iran has been working for some time on a new nuclear reactor in the town of Arak. Iran says the reactor is for nuclear research and medical isotope production, but as nuclear expert Mark Hibbs explained to me recently, it's designed in such a way that it would be "very efficient at producing plutonium," which could be used in a nuclear weapon after some other processing. Also as part this weekend's deal, Iran agreed to stop all work on the Arak facility.

7. I hear about how Iran wants a nuclear bomb to destroy Israel. Why would they do that?

This is a controversial point, both because it's such an obviously sensitive issue and because the only people who know for sure what Iranian leaders would do is the leaders themselves.

Most analysts, though, do not think that Iran wants to bomb Israel, or any other country for that matter. Iranian leaders can use some scary rhetoric at times, having referred to Israel as, for example, a "cancer." In action, though, they've behaved rationally and acted in their own self-interest. Setting off a nuclear bomb in Israel or anywhere else would absolutely guarantee the Iranian regime's immediate and total destruction. There's just no indication that Iranian leaders are anywhere near crazy or suicidal enough to do this.

Still, Iran is certainly hostile toward Israel (you could argue that it's mutual). Tehran actively supports the Gaza-based militant group Hamas and the Lebanon-based militant group Hezbollah, both of which have long records of attacking Israel and killing Israelis.

At the very least, a nuclear-armed Iran would feel much freer to use these militant groups to extend its influence and threaten Israel. So you can understand why Israel is especially concerned about Iran's nuclear program and wants the toughest deal possible. Arab countries along the Persian Gulf, particularly Saudi Arabia, have similar long-standing tensions with Iran and similar concerns about an emboldened, nuclear Iran.

8. So an Iranian nuclear weapon would be bad. What can the world do to stop it?

Not a lot. There are four bad options and one okay option.

The first bad option is to bomb Iranian nuclear sites, which probably wouldn't set Iran back more than a few months but would almost certainly make Tehran more likely to develop a nuclear weapon. This would also likely reduce international support for efforts to curb Iran's nuclear program.

The second bad option is a full-on invasion to topple the government. After the disastrous 2003 U.S.-led invasion of Iraq, a country that is much smaller and weaker, this is quite simply not on the table.

The third bad option is to try to topple the government in other ways. But the Islamic Republic has survived tougher things than we can throw at them. Even in Iran's mass 2009 protests after the disputed presidential election, people were calling for the government to behave better, not for it to collapse outright. Any Western-led effort to topple the government is probably just going to deepen its popular support and make Iran more likely to want a nuclear weapon to protect itself.

The fourth bad option is to try to force Iran to surrender and simply give up its nuclear program. This would be the status quo. International sanctions have indeed taken a tremendous toll on Iran. Meanwhile, U.S. and Israeli efforts to sabotage the Iranian program, such as through cyber-attacks, have found some real successes. But Iran's nuclear program has continued to grow despite all these efforts. The world, it seems, can slow them but not stop them.

That brings us to option number five: negotiating a deal directly with Tehran. This is the direction that the world is moving in with this weekend's agreement. The goal is that Iran gets its dignity-affirming nuclear program, but with enough restrictions and inspections and limitations that the rest of the world can accept it as peaceful. The downside is that this would be difficult to enact, no deal will please everybody and, most pointedly, it requires trusting Iran with a nuclear program even though it has cheated on past agreements. The upside is that all of the other options are much worse.

9. That was too much text so I skipped to the bottom. Did this weekend's deal solve all the problems?

Nope! This deal lasts only six months. If everything goes smoothly, the idea is that everyone will come back together and then sign a more permanent agreement. The deal is not perfect, as any workable agreement would have to be. Here are the bullet-points on the agreement and what comes next:

• Iran signed the deal in Geneva this weekend with a group called the "P5+1," which includes the five permanent (thus, "P5") members of the United Nations Security Council -- the United States, United Kingdom, France, China and Russia -- as well as Germany (the "+1").

• Iran has to "neutralize" its 20 percent enriched uranium, can't make any more uranium enriched above 5 percent and can't use any new centrifuges or build new nuclear facilities. This leaves Iran with the ability to run a peaceful energy program -- a real concession from the United States and other world powers -- but no more. The U.N. nuclear watchdog will make daily inspections at Iranian facilities.

• In return, Iran gets $4.2 billion in frozen overseas assets back, as well as sanctions reductions worth about $1.5 billion. This is not enough to save the Iranian economy but is meant to show Iran that cooperating is in its best interests. The United States and others promised not to impose any new sanctions for six months.

• Opponents of the agreement could kill it, and might try to. Israel openly opposes this deal, and Arabian Peninsula states such as Saudi Arabia are thought to privately oppose it as well. Some members of Congress are trying to pass new sanctions, which would violate the U.S. end of the agreement and likely kill it. Back in Tehran, hard-liners who oppose any compromise with the West could likewise scuttle it, perhaps by pressuring Supreme Leader Ali Khamenei to openly scorn the deal or to continue clandestine nuclear activities that violate the terms.

The best-case outcome here is that everyone accepts the deal in good faith and it works, leaving Iran with a purely peaceful nuclear program and healthily functioning economy, thus increasing lucrative trade and reducing the risk of conflict between Iran and the West. The worst-case scenario, as Israel warns, is that Iran exploits the deal to weaken international sanctions -- and perhaps break apart the U.S.-led international coalition to prevent a nuclear Iran -- while also developing a nuclear weapon.

The most likely outcome, though, may be that the deal falls apart, maybe due to diplomatic bickering over the fine print, maybe over more substantive procedural disputes, maybe under opposition from Iranian or American or Israeli skeptics. Maybe, as with the failed 2004 agreement, someone doesn't uphold their end. This is the Middle East, after all, where the status quo is usually a safe bet.