Halo 2's Matchmaking, designed to mirror the on-the-couch experience of playing with your friends in the living room, returns, retuned and upgraded for Halo 3. Since the Halo 3 Public Beta, some things about Matchmaking have changed, some things remain the same. Here's what Bungie said then, updated where appropriate for Halo 3:
In Halo 3's Matchmaking system, you'll find a ton of new options and abilities. Bungie's made their best effort to improve the already social nature of Matchmaking by giving you some new powers while you use the system to find games and opponents. Now it's easier than ever to meet people you like, with similar skills and experience.