...what does wikipedia say?

A quesadilla (pronounced kay-sah-Dee-yah|IPA kej.sa.'di.ja) (literally, "little cheesy thing," from Spanish "queso", meaning "cheese", plus "tortilla", and resulting in the mixture of both for "quesadilla" not using "o" for a Spanish linguistic rule) is a dish in Mexican cuisine that involves cooking ingredients (most populary cheese, inside a corn or wheat tortilla or a wrapping of masa dough). Exactly what constitutes varies from region to region and between the US and Mexico, and is not universally agreed upon by aficionados and chefs, but there are certain commonalities between the different versions that people generally agree upon, namely that the quesadilla is cooked after being filled or stuffed, while a taco or burrito is filled with pre-cooked ingredients. The purist faction may argue that only the folded-masa, empanada-style Mexican version is a "real" quesadilla, but some well known chefs such as Rick Bayless make more liberal interpretations.