Quesadillas are just another of the seemingly endless variations of dishes that can be made using tortillas (either flour or corn varieties). Although authentic Mexican quesadillas are usually made from corn tortillas, in the Tex-Mex variation and also in Northern Mexico, flour tortillas are much more common.
The concept is very simple - place the ingredients of your choice in one half of a tortilla and heat in a frying pan or on a comal - a Mexican cast-iron griddle pan. Then fold over and cut into wedges and eat! In Tex-Mex cuisine, 2 tortillas are often used instead of 1. In Mexico this would be called a sincronizada (synchronised) rather than a quesadilla. Confused?!?!