We ate at Tostadas Coyoacán - the original tostadas place in the market. The best way I can describe a tostada is like the hard, yellow taco shell that's sold in the U.S. (and doesn't exist in Mexico), but flat and circular. You can put a number of ingredients on top of the tostada: chicken, chicken with mole, tuna, steak, shrimp, ceviche, etc. Depending on what you choose, it might have lettuce and cream (it's not sour cream and it's not a sweet cream, it's just.... crema) on top as well. It's generally messy and delicious. The guys at Tostada Coyoacán work so fast preparing the orders - it's impressive to watch them.
Here are a few photos: