The quesadilla is one of the first 'international' snacks I remember learning to make. When my brothers and I got home from school, we would have things such as instant noodles, rice crackers and dried cuttlefish - snacks that Asian children around the world seem to eat. But two houses away from ours lived the Valdivias, and when I visited them, their children would be making Mexican snacks including quesadillas.
At its most basic, a quesadilla is shredded or crumbled cheese layered between two flour tortillas (or one folded in half) that's grilled in a lightly oiled pan. The Valdivia family made them better than most I've tasted since then, because they pan-fried the quesadilla in melted butter, which has a lot more flavour than oil. A Mexican grandmother might throw up her hands in horror at the use of butter, but the Valdivias liked it, and so did we.
The type of cheese for quesadillas is very important - it has to become soft and gooey when heated. The Valdivias used queso fresco or Monterey Jack, and occasionally they served the quesadilla with home-made salsa. But, please, don't get overly elaborate with quesadillas - avoid using cheese that's very strongly flavoured (I tasted a version that had been made with goat cheese, which was just plain wrong), and don't add too many toppings.