“Steam Family Sharing” is a new service from Valve, which permits Steam users to allow family members and close friends access to their library of games.
The main advantage of this is that it allows family/friends to access your Steam games, without having to physically kick you off your own PC. If my brother wants to play some Bastion, he will now be free to do so on the lounge computer.
Another big advantage is that separate save files are made on the shared computer, as well as separately earned achievements.
What isn’t allowed, however, is two users playing games from the shared library simultaneously, as confirmed by Valve in their FAQ:
Can a friend and I share a library and both play at the same time?
No, a shared library may only be accessed by one user at a time.
While it would be nice to be able to have simultaneous access to a single account, I can see why Valve had to draw the line here. The “lender” will always have priority, and be able to access their library at any time:
When I authorize a device to lend my library to others, do I limit my own ability to access and play my games?
As the lender, you may always access and play your games at any time. If you decide to start playing when a friend is already playing one of your games, he/she will be given a few minutes to either purchase the game or quit playing.
For those of you with younger siblings, or children, I’m sure you’ll appreciate this new service. The beta will launch next week!