A co-op multiplayer game is any title where you collaborate with other players towards a shared goal. This can be as gentle and wholesome as building a farm in Stardew Valley or as frantic and pulse-pounding as fighting alien hordes in Helldivers 2. What's important is that the experience is enriched by the presence of your fellow players.
The titles on our list of the best co-op games are all great examples of multiplayer done right. Though co-op titles come in a lot of different flavors, we've confident that you'll find something to your liking.