What's a Dungeon?

I just started playing and I'm wondering what a dungeon is? How is it different than another areas?

Dungeons are instanced areas that have bosses and generally have better loot drops than regular mobs.

The enemies in dungeons are much harder than a normal enemy of that level, termed "Elite" enemies. They tend to come in groups, and usually require a group of five people or a much higher leveled character to kill them off.

A dungeon being instanced means that each group who enters the dungeon has its own full copy of the dungeon at their disposal, instead of having to fight with other groups over the mobs (enemies). Its a way to get better loot easier, although the experience is usually lower compared to running quests. Most of the lower level dungeons start out pretty easy and short, but as you get farther in the game, they get harder and longer. I've had several experiences when dungeon runs last almost four or five hours.