Mad Moxxi's Underdome Riot is the second piece of DLC for Borderlands. It features three new riot arenas (Hellburbia, the Gully and Angelic Ruins) and storage for players' items. The plot of the DLC is Moxxi, a crazed lover who is setting out to find her 4th husband, leading her to make the arenas in the DLC. Players fight several of the game's enemies, including bosses, in arenas. No experience is gained from killing enemies in the arena battles, but experience can be gained from completing challenges or quests in the arena. New game modes are added, such as low gravity fighting, enemy health regeneration, and shieldless fighting.
Source: Wikipedia, "Borderlands (video game)", available under the CC-BY-SA License.