The recipe for quesadilla (pronounced kess-a-dee-ya) changes as it travels. The basic idea is there. It has an outer crust, soft or hard depending on the location, called a tortilla. This is folded, in a half-moon shape, around an array of delicious ingredients (mushrooms, vegetables, beans, chicken or beef are favorites), all held into place with melted cheese. Salsa and/or guacamole are usually offered for inclusion too. Nevertheless, outside Mexico, quesadilla rarely tastes the same.
Often this is due to the little changes of convenience and culture made beyond our borders. For example, in Mexico, the salsa is often a matter of honor and pride. It may be a family secret recipe, handed down from a succession of grandmothers, and experimented with until perfection is reached.
In other countries, salsa is just salsa with none of the judgmental values attached. It's ok to just reach for the mass-produced jars of chopped tomato with flavoring and still call it salsa. A Mexican cook would never get away with that. They wouldn't dare produce a bland salsa and hope to escape with their culinary reputation intact.
Also Mexican tortillas tend to be made out of corn. They are rolled by hand and individually formed over smoky comals. In America, Canada, Australasia and Europe, the tortillas are generally made from wheat flour and fried or baked as a batch.
It all comes down to what works and what is acceptable. Corn is more plentiful and cheap in Mexico (it is one of our major exports) than wheat; while the reverse is true for other nations. Those little taste details, as with the salsa, would pass without remark elsewhere, so it makes sense for businesses to take the cheap and lazy option. But all of these things alter the quesadilla too, as it is created across the world. Hence the assertion that, for the real deal, Mexico is the only place to eat one.
Quesadillas are incredibly popular here. Just about every street corner, mercado and food court has vendors selling them. No self-respecting restaurant and cafe would be without them on the menu. The savoury smell whafts out drawing locals and tourists alike. A wedge of quesadilla can be a light snack, to see the hungry through their journey towards their own kitchen. A full-sized portion can (and often is) the full meal itself.
It is universally savoury here, though some have experimented with sweet quesadilla. The 'quesa' part of the name comes from the Mexican word for cheese, 'queso'. Oaxaca cheese is usually used here (another taste difference, as Monterrey cheese is often exported for quesadillas elsewhere). This is white and arrives semi-hard, until it is softened over the comal. Then the cheese becomes stringy and melts with a taste similiar to Italian mozzarella.
The fillings are limited only by imagination and availability. Chicken and beef are standards, though other cooked meat or seafood, like shrimp or crab, can be used. For the vegetarians, many a tasty quesadilla has been made with mushrooms, refried beans and vegetables. So much may go into this mix. Seeds, stew, mole... the list goes on. Just ask and it may happen.