It's a fun, free-to-play MMO where there's two sides, the Criminals, who rob stores, steal cars, etc., and the Enforcers, which are essentially the cops.
The game was originally a pay-to-play game called APB: All Points Bulletin (Since that's what APB stands for) and was released in June of 2010, but the servers were shut down in September of 2010. Then it was bought by another company and was re-released as Reloaded in May of 2011.
Anyway, the game is similar to GTA, as you can steal cars, run over pedestrians, and other things like in GTA. There's also a group system, that either puts you into a specific group that you choose, or it can automatically put you in a group. Groups do missions together, which makes it a lot easier then when there's just one person.
Anyway, it's fun, and you should at least try it. Here's the site:
http://www.gamersfirst.com/apb/
The wiki:
http://www.apbrwiki.com/
And I suggest you download it from Steam as it probably downloads faster and doesn't install a bittorrent client like the installer from the main site does.