Riot Mode is the new mode, introduced with the DLC Mad Moxxi's Underdome Riot. It consists of horde after horde of enemies (think Horde from Gears of War 2, survival from Left 4 Dead 2, Nazi Zombies and Firefight from Halo 3: ODST) that gradually grow tougher and tougher.
It's a lot easier to do it with four people, and take note: The level of the person that is hosting will be the average level of the enemies.
Along the way, individual fight rules will be introduced into your battles, either as a benifit or a handicap, such as triple health or weaker guns, and any player who bleeds out respawns but only in a cage high up, where they can only help slightly.
Each Arena has a different theme, for instance, one of them has ALOT of skags while others do not.
There is the smaller arenas which contain 5 round each with 5 waves each and then the larger ones with 20 rounds and also 5 waves each.