Char siu bao is a bun (bao) with a roasted pork (char siu) filling.  There are two varieties of these buns, one is baked, and the other is steamed.  The former is like a jelly doughnut but with meat, the latter is like a moist cake.

The steamed variety are the kind you find in tea houses when you are out for dim sum, the baked buns can be found in bakeries.  Char siu is not the only fillings for these buns, sometimes they are made with chicken, beef curry, or even coconut.