From Halopedia, the Halo wiki
Juggernaut is a multiplayer gametype available in Halo: Combat Evolved, Halo 2, Halo 3, Halo: Reach and Halo 2: Anniversary. It was originally a version of Oddball in Halo: CE. This was later changed.
Juggernaut is the "Halo" version of tag. Although it resembles Slayer in that one must amass so many points to win, there are two major differences.
The Juggernaunt's purpose is to kill as many players as possible in order to amass points. The other players' goal is to kill the current Juggernaunt, in order for them to become the next Juggernaunt. To counteract the Spartans and Elites out to kill him, the Juggernaunt is sometimes aided with overshields, Active Camo, increased speed, increased damage, increased damage resistance and infinite ammo (not bottomless clip).
Halo 2: Anniversary