Steam Family Sharing has just gotten out of exclusive beta status and Valve is now making it available for everyone on Steam.
Steam Family Sharing allows users to share their games libraries to other authorized users. Up to 10 devices and 5 users can be authorized to play your games. All shared games will allow each account to earn achievements for their own profiles.
Do note that there are some limitations, one of the biggest is that the sharing account CANNOT be logged in while in use by another account.
More information can be seen in the official page.