Muslims are Muslims are Muslims to most of us. But, there's clearly a big deal about the differences between the two because they've sure been fighting one another for a very long time.
So, what IS the big deal?
Who succeeded Muhammad?
That's it? That's all there is? And it's worth slaughtering tens of thousands of people?
Well, the story is @ Differences between Shia and Sunni Muslims - Business Insider along with a very neat interactive map on how religions spread across the world.