In Halo: The Master Chief Collection (MCC), most story-based modes can be played either single-player or with friends. Playing any of these single-player modes with friends can be referred to as playing cooperatively, or "co-op." However, it is most common for players to refer to playing Campaign missions with a friend as “playing co-op.”

The difference between co-op Campaign, Firefight, and Spartan Ops can be confusing for newer players. Here is an explanation of the differences:

Co-op Campaign

Co-op broadly refers to having more than one player in the game cooperating (you can’t “cooperate” without a friend around to cooperate with). You can play co-op in any of the Campaigns, Firefight, and Spartan Ops.

When someone says “playing co-op” without specifying any mode in a Halo game, they are usually referring to playing Campaign missions with a friend.

• Halo: Combat Evolved and Halo 2 support co-op Campaign up to two players.
• Halo 3, Halo 3: ODST, Halo: Reach, and Halo 4 support co-op Campaigns up to four players, but only two players can split a screen at a time (splitscreen is Xbox only).

Firefight

Firefight is a game mode in Halo 3: ODST and Halo: Reach where up to four players can fight against waves of attacking Covenant enemies.

• If you play single-player Firefight, it’s just you against the attackers.
• You can play co-operatively by using Social Matchmaking or by hosting your own Custom Firefight game and playing with the players you invited. They would be visible under “My Game Session” in the top right before starting the match.

Spartan Ops

Spartan Ops is a game mode in Halo 4 where up to four players can work as a team to perform various objectives while using their customized Halo 4 loadouts. You can play Spartan Ops co-op, but you can play it single-player instead if you prefer.