What sharkenleo described is called queso flameado which is the closest Mexican dish to queso as we know it.

In the north of Mex queso flameado is usually a queso asadero (hence the name) or a queso fresco that is seared or even grilled to a plyable consistency and served that way. But they also call the Oaxaca or Chihuahua melt flameado too.

But traditionally in most of Mex the melt of cheeses with chorizo, mushrooms, and exotics like huitlacoche is called Queso Fundido, as it is truly a bubbling scalding pan of goodness.

Equally true quesadillas are masa stuffed mini tacos or empanada like dealies that are deep fried, talk about good.

No mention of sodium citrate? Blasphemy...