The tostada was created when tortillas went stale but were still fresh enough to eat. Not wanting to waste old tortillas, which was one of the staple foods of the Mexican people, beans, rice, meat, cheese and vegetables were spread onto the tortillas like an "open faced" taco. This invention became very popular and people soon began to fry fresh tortillas to recreate the dish.
A tostada is often served as an appetizer typically topped with a thin layer of refried black bean paste (frijoles refritos), chicken or beef strips or other kinds of animal products. These are usually topped with thinly chopped lettuce strips, sour cream, chopped onion, salsa and guacamole or sliced avacados. As a general rule, due to the flat construction of the tostada, the main topping (i.e. bean paste or meat) must be sticky or pasty enough to stay on the tostada. This helps prevent the other toppings or garnishes from falling off while it's being eaten.
In Colombia, tostada refers to a green, unripe plantain which has been cut into sections, fried, flattened, fried again, and salted. These are also known as tostones in other parts of Latin America.