Steam Announces Family Sharing

Steam announced a way with which you can share games in your Steam Library with "close friends" and family. You will soon be able to authorize a limited number of computers (10, according to Valve) to play certain games, each with its own Steam Cloud saves and achievement progresses.

Something like that would allow you to keep your game progression, unlocks, and achievements, separate from those of the people you're sharing your games with, who can keep their own stuff. The method is literally cartridge-sharing in the digital download era.

Here is Valve's explanation on how the system works:

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 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.

A lot more of your looming doubts are answered in this FAQ.