Valve has announced that the first 1,000 participants have been sent an email message inviting them to Steam Family Sharing beta.
The service, which was announced last week, is a new feature that “allows close friends and family members to share their libraries of Steam games.”
This means that your friends, family, roommate etcetera can play any game in your game library with your permission.
They will need to be logged into the same machine as your account at first, so that you can activate a link between the two Steam accounts which will allow your library to appear remotely through their game list.
Once a device is authorized, the lender’s library of Steam games becomes available for others on the machine to access, download, and play. Though simultaneous usage of an account’s library is not allowed, the lender may always access and play his games at any time.
If he or she decides to start playing when a friend is borrowing one of his games, the friend will be given a few minutes to either purchase the game or quit playing.
You can find out more through the links.