From Halopedia, the Halo wiki
While prior Halo games featured power-ups in the form of Overshield and Active camouflage, these on-map pickups were activated immediately upon acquisition. Halo 3 introduced equipment as a new addition to the "golden triangle" of Halo gameplay alongside weapons, grenades and melee. Equipment can be found in campaign and multiplayer as pickups in the game world - once picked up, they can be activated at any time with a single button press. Equipment are single-use pickups and only one such item can be carried at a time.
In Halo 3, equipment can be found in campaign and multiplayer modes. In the campaign, equipment can be employed by the player and by Brute AI enemies, with several equipment pieces reserved for the campaign not available in multiplayer. Most equipment can be found in multiplayer placed on the map like any other power weapon. The mainstay list of equipment found in multiplayer includes the trip mine, bubble shield, deployable cover, flare, gravity lift, power drain, regenerator and radar jammer. The campaign features all of the prior-listed equipment with the addition of the Invincibility, Cloaking and Auto-Turret.
The Invincibility equipment can only be found by killing Brute Chieftains, while the Cloaking can be found on the bodies of Stalkers and on the map in some areas. The Auto-Turret is exclusively found in a handful of levels and can only be used by the player. The Auto-Turret has since seen its addition to multiplayer with the release of Halo 3 on PC as part of Halo: The Master Chief Collection, and can now be spawned in Forge mode.
In Halo 3 Forge, equipment can be used to overload a map, allowing the player to escape the map's confines.
Halo 3: ODST
Almost all equipment - with the exception of the gravity lift, auto-turret, regenerator and radar jammer - can be found again in Halo 3: ODST. Unlike Halo 3, equipment cannot be used by the player, and only exists within the game to be used by Brute enemies against the player.