Users considering a purchase on Steam may now browse reviews from friends and the community, leave comments on reviews, and indicate which reviews they found helpful.
Once a user has played a game or used a software title on Steam, they are able to write a review of that product and share their experience with everyone on Steam.
Steam Reviews is an evolution of the Recommendation system that has been a part of Steam since 2010. Over the past few years, Steam users have written over 7 million Recommendations of games and software to their friends. Those Recommendations have been upgraded to Steam Reviews. As such, users may now choose to make their past Recommendations visible as Reviews to everyone on Steam, if they wish, by visiting their list of Reviews and changing the settings accordingly.
As of right now, it seems as though the reviews are still simple a "yes/no" type system where you say if you recommend the game or not. Additionally, you don't even need to own the game you're reviewing but you do need to have at least launched it on your Steam account. This means that any title shared to others via Family Sharing can be reviewed by them if they play it.
A full FAQ and details can be found at the Steam Reviews information page.