The map, from Columbia University's really exceptional Gulf/2000 Project, shows the different ethnic and linguistic groups of the Levant, the part of the Middle East that's dominated by Syria, Lebanon and Israel. Each color represents a different group. As you can see, there are a lot of groups swirled together. There are enclaves, and there is overlap.
Ethnic and linguistic breakdowns are just one part of Syria's complexity, of course. But they are a really important part. The country's largest group is shown in yellow, signifying ethnic Arabs who follow Sunni Islam, the largest sect of Islam. Shades of brown indicate ethnic Kurds, long oppressed in Syria, who have taken up arms against the regime. There are also Druze, a religious sect, Arab Christians, ethnic Armenians and others.
Syria is run by Alawites, a minority sect of Islam whose members include President Bashar al-Assad and many in his inner circle. They're indicated in a greyish green, clustered near the Mediterranean coast. Although Alawites make up only 12 percent of the Syrian population, they are playing a crucial role in the war, fighting to prop up Assad's regime.
There are a couple of ways to think about what this map says about the Syrian civil war, beyond the strategic implications of where Assad is strongest (along the Alawite-heavy coast) and where he's weak (in the Kurdish regions, for example).
The first is what you might call the Fareed Zakaria case for why Syria is imploding (he didn't invent this argument but is a major proponent). Zakaria starts with the premise that Syria, like many other Middle Eastern (and African) countries, has highly artificial borders that were created by European colonial powers. Those powers also tended to promote a minority and rule through it. This tactic badly exacerbated some preexisting sectarian tensions. It also forced countries into unsustainable power imbalances, with minorities ruling over majorities. That's not actually how Assad came into power -- his father seized it in a coup -- but Zakaria's thesis is that what we're seeing in Syria is in some ways the inevitable re-balancing of power along ethnic and religious lines, with the Sunni Arab majority retaking control from the Alawite minority. He compares the situation to post-2003 Iraq, when members of the Shiite majority violently took power from the Sunni minority that, under Saddam Hussein, had ruled them. That would explain why so much of the killing in Syria has been along sectarian lines. It would also suggest that there's not much anyone can do to end the killing because, in his view, this is a painful but unstoppable process.
The other way to look at this is that it's a war first and a sectarian conflict second. Religious and ethnic antagonisms have been around for many, many generations in the Levant, including Syria. Maybe what's happening is that the war began for political reasons -- people protesting dictatorship, the dictatorship overreaching in suppressing those protests by force, things spiraling out of control until it's civil war -- but that the fighting is causing people to retreat to sectarian identities and antagonisms, to make the old divisions deeper and more vicious. Sectarian conflict, after all, can have its own self-reinforcing logic: Alawites are bonding together in part because they fear, not without reason, that they'll be slaughtered in Sunni revenge killings if Assad loses. Sunnis see Alawite militias forming and thus perceive all Alawites as their enemies, so they start attacking members of that religious sect, which makes other Alawites more likely to form in-group militias. And on.
Of course, something as complicated as sectarian conflict in a country with many religious and ethnic groups could never really be defined by one neat theory. There are likely many different factors behind what's happening. But this map is a helpful way to start understanding it. The version up top is actually cropped; the full-size version is below. Click either to enlarge them.