A dungeon represents one of the possible areas where a roguelike game is played. Dungeons are a subterranean maze of rooms interconnected by corridors and stairways, where each Level is progresively more dangerous and has greater rewards.
There are some hardcore roguelikes in which all or most of the action happens inside a dungeon, thus increasing the "Dungeon-Hack" factor.
The first roguelikes were centered around dungeons, which were first represented as rectangular rooms joined with passageways, and had a horizontal division of levels, connected by ascending and descending stairways.
In some other games, there's a wilderness or some other kind of overworld, from which you can get to different dungeons.
Dungeons are the oldest class of roguelike maps, and are favored for their easy generation; they are present in almost every roguelike, either as the main action stage or as complementary areas.
The nature of dungeons as playgrounds full of monsters and treasure that increase in reward as the game advances, has spawned a subgenre of crpg that is known as dungeon hacks, in which the storyline is not very important, and which sometimes promotes the munchkisism, some roguelikes, including some of the big ones, may be described as dungeon hacks, altough the tendency nowadays is to look for another alternatives and let the dungeon hack as a complementary feature.
In real life, a dungeon stands for the strongest tower to be found inside a castle, usually used as a last defense stand or to keep prisoners.