Anna Sweet from Valve explains:
We've been working on this feature for the last few months with the input from a group of indie partners, and the response has been extremely positive. With the additional help of beta testers, we are able to launch with a solid line-up of titles for the community to start viewing and rating. And, as we've done with all Steam features, we intend to continually grow and modify Greenlight as more and more developers and community members have a chance to get involved.
Steam Greenlight works by letting developers and publishers post bits and pieces about their game. The aim, of course, is to convince the Steam community that their game should be published on Steam - the games with the most votes win!
Described as a bit of a "clearing house" for game submissions, Greenlight is also well positioned to provide a whole bunch of added exposure for new games, as well as giving developers and creators a new opportunity to interact with fans (both current and potential!).
For more information - or to check out the games that are already on the list, head to www.steampowered.com/greenlight.