Once someone shares their Steam library with you, you’ll be able to play and download the titles as much as you want and even get access to your own cloud saves and achievements. The service is enabled by a simple authorization process.
There are a few small limitations here, a single Steam library can not be accessed at the same time by two users. The lender holds precedence and can access his or her games at anytime. If the lender decides to access their games while another authorized individual is using them, the other friend will get the boot with two choices to either just quit the game or purchase it themselves.
“Our customers have expressed a desire to share their digital games among friends and family members, just as current retail games, books, DVDs, and other physical media can be shared…Family Sharing was created in direct response to these user requests.”
Additionally, there will be a limit of 10 authorized users per library, again very similar to what Microsoft originally announced for the Xbox One. Another caveat is that not all the titles available on Steam will support Family Sharing, due to technical issues, games that require third party authorization keys, accounts or some kind of subscription will not be supported.
The new feature will be made available sometime next week, in limited beta. Head below for more info on the Family Sharing service and on how to get into the beta.