It fucking rocks. That's all you need to know.
Ok, but really the rundown is what has been said already.
You pick the gametype you want (TDM, FFA, Search and Destroy, Team Tactical (2v2 or 3v3), Domination (like territories), etc.) A cool thing is that if you're playing alone and don't feel like getting into a game where there's a team of guys actually working TOGETHER leading to your team getting owned, you can pick non-party variations of the team modes.
Once that's done, the maps are cycled through and like RoboJ said, a majority vote would skip the map. The downtime between games is much less than Halo 3, too.
In summary, my life is fucking over when COD4 comes out.