Online multiplayer gaming sessions can be arranged in several ways. The traditional approaches are gathering a group of friends into a lobby or server to play, or browsing a server list and joining a game in progress.
Matchmaking is a third option, which gives players the ability to get into a game with less effort. Individuals or teams search for a game, and are matched by the system with other similar players. Once an appropriate number of players is found, the match is made and the game can begin.
The main benefits of matchmaking are more competitive matches, the ability to search as a team and play another team, and variety in game configurations. The trade-offs include a long search time to find a good match, and being unable to choose the exact map or gametype to play.
Microsoft's Xbox Live service popularised matchmaking for console multiplayer games, with its flagship Halo 2 title being the first to use a ranking system and a variety of playlists. Other major games have gone on to adopt this approach, and it is now the most popular method of arranging online games on consoles.
Many matchmaking systems make use of skill estimates, or ranks. This allows players of similar ability to play against each other, giving closer and more competitive games for all skill ranges. TrueSkill is a Microsoft Research technology which attempts to meet this goal.
Playlists are also associated with matchmaking. These are used in highly configurable games to divide the overall possible playing field into clusters of similar game types. This allows the player to have some choice in what type of game to play. In online shooters, playlists include several configurations of rules and maps along a theme.