In this two-minute video, reporter Karen DeYoung and The Washington Post's video team give a very brief history of the Sunni-Shiite divide and what it means for Iraq's escalating violence today. It's important to note that this religious division is one of many factors driving the conflicts in the Middle East. Although theological differences are not in themselves enough to explain the fighting, it's important to understand the very basics to grasp what's happening in the region.
Here, to illustrate the Sunni-Shiite divide, is a map showing the religious groupings in the region.
As you can see, Sunni and Shiite are spread out enough that they have to coexist within their respective countries, typically with one group in a majority and the other a minority. But they're also clustered enough that groups of Sunni and Shiite can develop local power bases that can compete with formal government authority. It's not ideal.