Quake Live is an updated version of Quake III Arena. It has "over 25 arenas and 4 game modes," including a multiplayer mode for playing live against other players, and an "offline" mode for playing against bots. Because the game is hosted online, this means that id can continually tweak the game over time--even after it is officially released--similar to how Linden Lab is always making iterative improvements to Second Life. Unlike how Second Life works, however, there will be no opportunities in Quake Live for users to perform any micro-transactions.
"As a beta tester, we are asking you to help us test the game delivery mechanism and core game technology, and to provide feedback on gameplay and game levels. The servers are always available, but you are encouraged to join us for our regular focus tests and special events (you will receive email notifications).
Please note that QUAKE LIVE is a work in progress and your experience as a beta tester does not reflect the final set of functionality that will be available. We encourage you to post comments, issues, suggestions and other feedback to the QUAKE LIVE forums."
There is no official word on when Quake Live will be officially released, but some speculate that it might come before or at id's QuakeCon 2009 event, which runs from August 13 to 16 in Dallas, Texas. Others think that id might take a more Google-like approach, and leave Quake Live as a perennial beta. Even with the beta status, that didn't stop id from sponsoring a Quake Live tournament last year at QuakeCon 2008. There will another Quake Live tournament at this year's QuakeCon as well--whether Quake Live will still be in beta at that time remains to be seen. You can sign up for the Quake Live beta program here.