A dungeon is a place left behind by a previous civilization, race, or group that wanted to leave stuff behind for whatever reason. All contain loot, some contain ancient evils, and some are coming of age trials. A dungeon typically contains traps, monsters, and of course the reason people don't just block them off forever, loot.
While [Mages] study which empire had built a dungeon and what their composition is, Adventurers have thought of a simple system to categories them. A dungeon is usually either intended for containment, future generations, or as an act of vengeance. There is also study on how they defend the dungeon, such as horde type, boss type, or trap type.
These dungeons are meant to contain something. It could be a threat, a natural resource too valuable to let someone else get ahold of, or something completely random. One such dungeon, for instance, had been built solely to combat Vampires and therefore used mostly light-based traps and spells.
Dungeons built with traps that test people entering. For example, an empire is about collapse. Before it comes to that, however, they desire to leave something behind. They wouldn't like their valuable artifacts going into the hands of just anyone, though; only those that have proved themselves should receive them. As a result, they built one only the best can enter and survive.
If an empire or civilization is upset at the thought of future generations to take what was once in their possession, they build a dungeon to seal away their most valuable treasures. Then, they fill it with the nastiest traps and monster eggs. So if adventurers were to go into them, they would be attacked by an army of monsters that attempt to eat them. Such dungeons are designed to kill whoever dared to enter it.
- Dungeons traps claim more lives than monsters most of the time.