... with the same ingredients but with a twist.
Tacos are made with tortillas rolled or folded around something, with quesadillas you put the raw dough of the tortilla and then put something inside, then fold, and then fry it. A variation would have the lady making special tortillas, a bit ticker, on the spot and then stuff them.
But another difference from tacos is the ingredients you put inside, traditionally you have meltable cheese (Oaxaca and asadero are the most common) and a variety of already cooked items that could be a main dish by their own, like chicken, chicharron, mushrooms, squash flower (Flor de Calabaza), corn fungus (huitlacoche, a delicacy in Mexican tables only available on the rainy season), cow brains (sesos), and lots more.
Quesadillas are made on the spot, so you will always have a freshly made one. As most of the food, you need to eat it while still hot, after an hour it won´t taste the same and the texture will definitively be lost, no microwave oven or Granny tricks will help in this department, if you don´t have your quesadilla on the spot you miss part of the experience.
One fact that may surprise you, is that normally there is only one lady (men are bad for shaping the quesadillas) preparing the quesadillas. Even on a very crowded food stand, the lady may have some help on other areas, but cooking is Her position. And they can have orders of 20 or 40 in a matter of a short time, so you need to be patient, this is not a McD (fortunatley) where food is made fast, true it´s junk food but more tasty.