Game developers and publishers will post information and media about their games on Steam, while the community will check out them and decide whether they want to see it released on Steam or not. Users rate and leave feedback on games just like what they do to user creations on Steam Workshop. Sounds like a good idea, because this means game developers and publishers will have to work a little harder in convincing the community to greenlight their games if they want to see them distributed via Steam.
Steam Greenlight will be released on August 30. Check out the Greenlight website for more details.