People watch a broadcast of President Obama at a community center on Thursday, Nov. 20, 2014, in Nashville, Tenn. President Obama announced executive actions on immigration during a nationally televised address. (AP Photo/Mark Humphrey)

Illegal immigration is one of those topics on which almost everyone has an opinion -- maybe several opinions. But when it comes to the facts, the number of people with a grasp on them is a lot smaller.

Donald Trump, a contender for the 2016 GOP presidential nomination, said last month that illegal immigrants coming across the U.S.-Mexico border are in large part "rapists" and "murderers." And in San Francisco, the shooting death of Kathryn Steinle, 31, allegedly caused by man who has been deported multiple times and convicted of a series of very serious crimes, has set off a new wave of concern about immigration and so-called "sanctuary cities" like San Francisco.

It's a lot. That's why there is a Part I and a Part II to this post. But we've tried to boil it down to the essentials for your next immigration-related debate.

If a person is in the United States illegally, why isn't he or she arrested and shipped back to his or her home country immediately? 

This is one of those places where the country's commitment to freedom and liberty get a real road test.

As mentioned above, immigration is often complicated. At least some portion of the people who do enter the country illegally or overstay the terms of their visitor or student visas also have legitimate asylum claims. Asylum is limited to individuals who can provide evidence that they have faced persecution or might be killed if they return to their home country. And U.S. law says that most people caught inside the United States should be given a chance to make those claims in an immigration court. (For more information on asylum seekers, see Tables 16 and 17 here).

Now, layer on top of that more than 445,000 people awaiting immigration hearings. Most of these people cannot make a successful asylum claim but might have some other legal defense such as proof of a U.S. citizen parent or grandparent.

Sanctuary laws received national attention in July 2015 after an illegal immigrant with prior deportations and a criminal history pleaded not guilty to murdering a woman at a San Francisco pier. President-elect Donald Trump has pledged to cut federal funding to sanctuary cities. Here's what they are. (Jayne W. Orenstein and Osman Malik/The Washington Post)

Unauthorized immigrants caught inside the United States -- or in some cases at the border -- generally get a hearing in one of the nation's deeply backlogged immigration courts. Wait times now stretch into 2019.

That means that federal immigration authorities also have to make decisions about whom to hold during that wait and whom to release and trust to show up again.

How many people does the United States deport each year?

The data reporting here lags a bit behind and, of course, varies from year to year. But on average, between 2011 and 2013, immigration courts ordered about 414,650 people out in one way or another. Here's the picture painted in the Department of Homeland Security's most recent annual immigration data report, for 2013.

Enforcement Statistics

What is a "sanctuary city," anyway?

The policies and practices differ in the estimated 60 sanctuary cities around the country. That list includes major cities such as New York, Los Angeles, San Francisco and Houston. But generally, when someone has been, for instance, arrested for driving without a license and then identified as an illegal immigrant at a jail in a sanctuary city, they must serve jail time for state charges or pay related fines. Then, they are let go.

Most of these cities have identified some set of guidelines or conditions under which federal immigration officials must be alerted before the person's release. Usually they are connected to what's on the person's rap sheet.

But some either don't have them or don't follow them. For a deeper look at the role that San Francisco's sanctuary-city status has played in the immigration debate since the Steinle shooting, see this.

[California killing inflames debate on illegal imigrants, 'sanctuary cities.']

Once an undocumented immigrant has been arrested for committing a crime inside the United States, why do sanctuary cities let them go? 

During the apex of the country's illegal immigration challenges, before the recession, law enforcement officials in some communities expressed concern about the practice of releasing these inmates after they had served time for state offenses. Some of those communities entered agreements to help federal authorities with immigration enforcement. This went on between 2004 and 2012.

These agreements allowed local jails to house undocumented immigrants after they had served time on state charges and bill the federal government for this service. Sometimes inmates were passed along to jails in other places without any formal notice to family members, then into the immigration court system for an expedited removal hearing. In many cases, people were returned to their home countries in weeks.

That program was widely criticized as a possible revenue stream for some local jails and a potential violation of international human rights accords. Some people were unable to communicate with embassy officials from their countries of origin or notify family members of their arrests, basically disappearing without explanation. Civil liberties groups called it a vehicle for racial and ethnic profiling. One Tennessee sheriff described it as part of his toolkit to "stack these violators like cordwood." In addition, more than one analysis of who was deported and what happened during that process showed that most were people initially arrested for minor traffic violations and who had no criminal record.

President Obama touted the fact that his administration had deported the largest number of people in U.S. history. (Read the more complicated truth here.) Meanwhile, immigrant advocates said all of this deeply damaged already-limited police trust in immigrant communities, making people afraid to call police or provide information. That, these advocates argued, was the real threat to public safety.

This is where sanctuary cities come in.

What happens in other cities?

After a series of changes, new programs and memos from the top that were supposed to assure that more of the nation's deportation apparatus got aimed at serious and violent criminals, the Department of Homeland Security is now asking communities to participate in a different program, this time called Priority Enforcement.

Priority Enforcement won't formally begin until later this summer, The Washington Post reported Tuesday. When it does, it will ask local law enforcement agencies to notify federal immigration authorities before the scheduled release of an immigrant targeted for deportation. Those targeted for deportation include people with violent and serious crime convictions. And federal officials told The Post that they did make just such a request to the folks in San Francisco.

Finally, is there any evidence that those who enter the country illegally commit more crime than others?

The Fix looked at this issue this week and found an answer that shouldn't really be surprising.

Like every population, there are some people who have immigrated to the United States illegally who go on to commit serious and misdemeanor crimes in this country. But immigrants of all kinds are actually less likely to commit crimes than those born inside the United States. (For more detail check out this post immediately.) Here's the big take away in one chart.

Crime and Immigrants

This chart highlights all immigrants, but it's important to note that more than one-quarter of all immigrants currently in the United States are undocumented. So a spike in their crime rate would likely mean the "first generation" line wouldn't be so low.

OK. We hope that helps.