Dungeons are designed for a party of up to five adventurers.
A "valid" party consists of one tank, one healer, and three damage dealers (melee or ranged).
Players can travel to the dungeon themselves (or be summoned with a Meeting Stone or warlock's [Ritual of Summoning]), or they can use the Dungeon Finder tool to queue for specific or random dungeons.
Dungeons contain mostly elite creatures, which are more powerful than normal creatures.
Dungeons are more challenging and take longer than quests, but are not as difficult or as time-consuming as raids.
All dungeons starting with those from The Burning Crusade feature heroic modes, which are more challenging and contain better loot than the normal counterpart.