LAN or system link games are handy for multiple console homes, dorms, or apartments. Players will link up their systems to play a game of local-kind-of co-op in groups. This style of co-op is the most common on PC games and handhelds for this gaming generation.
Drop-in/Drop-out co-op is a handy little feature which allows players to jump in or out of a game as they see fit. Ways this is handy: When a co-op pair are close to completing a game, but one player has to go to work/school/sleep, that player can drop out and allow the other player to complete the progress of the game. Or, when you pick up a slimeball co-op partner via Xbox Live, Playstation Network, or Steam matchmaking, you can leave the game without penalty or canceling the slimeball's game, and vice-versa. Players that arrive late to the party can fill an empty slot, or players that have to leave early can open up a new slot without dropping the entire campaign.
Co-op Specific Content
Co-op specific content is a specific mode devoted to co-op which does not apply to the single-player or storyline of the game. These modes are often: Survival, where your team is set up against wave after wave of enemies. Co-op only campaigns, where you'll play with some friends through scenarios unrelated to the main storyline or campaign.
Single Player Content Available In Co-Op
With this feature, we want to see all of the single player content available in the co-op. While ideally this includes all of the cutscenes and story based elements, it's not always the case. As long as the core single player gameplay and rewards are intact, we consider it the the same content. So while Gears of War is a prime example of a game that embodies everything good about co-op, Call of Duty: World at War still has the single player content intact even though it's presentation of it isn't perfect.