Valve has announced a Family Sharing system for Steam. Much like Xbox One’s own now-removed initiative, this will allow users to share their entire games library with upto 10 members of family and friends.
Users playing on a shared library will have unbridled access to every game (including DLCs), except those that require third-party login systems (like Origin, uPlay), or subscriptions. However, the library owner will always be given top priority, with the lendee having limited time to boot out (or simply opt to buy the game) if the owner signs in at the same time.
It is unclear if every game on Steam will be supported by the Family Sharing system, or if developers will be given an opportunity to opt out off the programe. Region restrictions will also apply and limit which games a user is able to lend or share.
Steam Family Sharing is currently in Beta. You can register for the beta here.