You are now able to authorize 10 computers, and 5 user accounts to share your game library with.
Steam has announced that its Family Sharing system, which allows friends and family members to share games temporarily with each other, is now out of beta and in its first full release. "Lenders" are able to authorize up to ten computers, and five Steam accounts, who can then request to "borrow" a game for a period of time. The lender can lend a game to the "borrower," who has free reign of it until the lender wants it back, and which point the borrower must return it or purchase his own copy. If the borrower does purchase their own copy, their progress is carried over.
Before you go out and try to lend all your games to your buddies, there is a considerable downside to the service. There is absolutely no way for users to authorize an account remotely, meaning your buddy has to come over to your house and physically log into his Steam account from your machine in order to become a borrower.
For family members though, it should work perfectly, as you'll easily be able to lend out specific games without having to log out of your own Steam account.
When the beta for Family Sharing opened last year, Valve said the service "is designed for close friends and family members to play one another's Steam games while each earning their own Steam achievements and storing their own saves and application data to the Steam cloud."
Games with an additional third-party key cannot be shared between accounts, and regional restrictions will remain the same.
Source: Steam Community