As its name suggests, Steam Family Sharing allows those with blood ties (or those willing to lie about blood ties) to freely share access to retail PC, Mac and Linux game releases via Steam. "Players who share computers can now also share their available libraries with one another, each earning their own achievements and saving their own game progress to the Steam Cloud," reads Valve's official description of the service. "It's all enabled by authorizing shared computers and the familiar accounts that log into them."
Full details on how to enable Steam Family Sharing can be found on the program's website. It's a quick process, which should leave you plenty of time to decide who is worthy of membership in your new virtual "family."