Steam’s Family Library Sharing is now released for all Steam users, allowing them to share their games libraries with up to 5 friends or family members across 10 different computers.
Family Sharing was first announced in September 2013 and it has been available for some users in in closed beta till now. The system allows family members and their guests to play one another's games while earning their own Steam achievements and saving their own game progress to the Steam Cloud.
A shared library can only be played by one player at a time and it cannot be used to circumvent region-locks. For more info, you can check the FAQ on Steam.