Grifball was introduced by the fan community in Halo 3 when the "Forge" mode allowed for the creation of new types of levels and games. It was not featured at launch, but was eventually patched into Halo: Reach, and will now be available at launch in the latest title.
Grifball is a unique type of game variant created by Burnie Burns of Rooster Teeth, in which two teams attempt to carry a ball or bomb into the enemy base. All players are armed only with energy swords and hammers. So essentially it's futuristic death rugby.