http://store.steampowered.com/sharing


TL:DR version. You can let people on up to 10 PCs total play on your library. Only one person can access a library at a time, so if you are playing anything no one else can access your library. Also you can't play a MP game with someone both using your account. Games with any 3rd party DRM can't be shared.


FAQ:

I want to try this! How can I join the Family Sharing Beta?

To express interest in beta participation, join the Family Sharing Group on the Steam community. The Family Sharing beta will begin in mid-September, when a thousand Steam accounts from this group will be granted access to share their Steam libraries. You’ll know you’ve been selected when you receive an email from Steam inviting you to try out the new feature.
How do I enable Family Sharing on my computer?

Family Sharing is enabled in one of two ways: You can either locally enable sharing in Account Settings, with Family Sharing & Devices, or remotely respond to a user’s Steam request to share your previously installed games via email.
Is there a limit to the number of devices I can authorize to share my Library?

Yes. A Steam account may authorize Family Sharing on up to 10 devices at a given time.
Can I share specific games, or do I have to share my whole library?

Libraries are shared and borrowed in their entirety.
Can all Steam games be shared with friends and family?

No, due to technical limitations, some Steam games may be unavailable for sharing. For example, titles that require an additional third-party key, account, or subscription in order to play cannot be shared between accounts.
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.
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.
Sometimes the games I’ve borrowed are unavailable for me to play. Why?

Borrowed games are only available on devices that have been authorized by the lender. Borrowed games will be unavailable on even an authorized device when the lender’s library is currently in use on another computer.
Who owns and can access the DLC and in-game content associated with a borrowed title?

A borrower will have access to the lender’s DLC, but borrowers may not purchase DLC for a base game they don't own. Any player may purchase, trade, earn, or otherwise acquire in-game content while playing a game, but in-game items cannot be shared between accounts. These items remain the property of the account that purchased or acquired them, whether borrowing or lending the base game.
Can region-restricted content be shared across regions?

No, any region restrictions will remain in place when lending or borrowing content.
Will I be punished for any cheating or fraud conducted by other users while playing my games?

Your Family Sharing privileges may be revoked if your library is used by borrowers to conduct cheating or fraud. We recommend you only authorize familiar computers you know to be secure.

I'm confused. You can install Steam on any machine you want. How is this any different than doing that and just letting your family sign in on your account?

This doesn't break the terms and agreements thingamajig?

If Steam's system is that "advanced" in order to block third party DRM games from family sharing, one would expect their store pages to be more precise on such information, no?

I am rather curious on how this feature will handle such titles. Added security when sharing with your "family".

...Maybe?

...guess we will find out soon enough.

Could this new feature be open for abuse? What are they trying to accomplish with this? More sales, perhaps?

You're already allowed to have your Steam games installed on several PCs/Laptops but so far you were not allowed to have others play your Steam games.
And as of course everybody was following this rule to the letter it is very nice of Steam to now allow you to have other people play your games on your computer.

Quote from the "subscriber" agreement:
" You may not reveal, share or otherwise allow others to use your password or Account."

Additionally, you're still on your own account, so you have your own separate achievements, config files, cloud saves, etc. If you decided to purchase the game for yourself you would already have progress in it.

I figure it would have been easier to just write it into the EULA and make an announcement. Same result, as far as I can see. Well, from what it says in the announcement you're still screwed if your 'authorized' friends/family get caught cheating or committing fraud, so I don't really see the added security.

"Can region-restricted content be shared across regions?
No, any region restrictions will remain in place when lending or borrowing content."

Wonderful ¬_¬

Yes. (H) 500+ games account, will lend it for a week. (W) $10 in paypal/GOG/Steam wallet.

From my understanding of the FAQ, you're only screwed out of your Family Sharing privileges, so your account would be safe from hijacking or a Steam ban.

Excellent point.
Since that is a significant part of what attracts people to Steam (and Live and PSN) that is indeed an important plus.

I would assume if you want to play a game from your library that Steam will give the other person a notice that they are about to be kicked out and give your games back to you. I'd guess it won't work in offline mode, but if someone is "borrowing" your games, you could play in offline mode. I'd guess this won't work for lenders unless they are online.

How is it different from me giving my account credentials to my girlfriend to use my account anytime she pleases as I've been doing for years now?