AMM

From The StarCraft 2 Wiki

The Automatic/Anonymous Matchmaking, also known as AMM, is a feature on Battle.net added in WarCraft III in 2002 and has been copied by many other game publishers into online games since then.

[edit] Functionality

This feature allows players to log on to Battle.net to play matches online within seconds, as the AMM system will match your skill level with other players and start a game for you. It looks at your win/loss ratio and ladder ranking when deciding a suitable opponent of the thousands of online players looking for a match. The system has been praised by fans for its functionality.

[edit] Arranged Teams

AMM can also be played with Arranged Teams. Many players find the AMM Arranged Teams to be even more fun than playing solo. It works in the same way, but incorporates for more players in the team. While many game publishers have copied the AMM feature, not all use the Arranged Teams addition, which can seem surprising.