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 Juggernaut's purpose is to kill as many players as possible in order to amass points. The other players' goal is to kill the current Juggernaut, in order for them to become the next Juggernaut. To counteract the Spartans and Elites out to kill him, the Juggernaut is sometimes aided with overshields, Active camouflage, increased speed, increased damage, increased damage resistance and infinite ammo (not bottomless clip).
Halo 2: Anniversary