What are Authorized Devices on Steam and how do I use it?

I noticed there is a section called "Authorized Devices" on my Steam Account Page.

Currently, it says only:

Manage and share devices

You can authorize up to 10 devices to allow registered users to access their library of games from them.

There are no authorized devices.

You can authorize up to 10 users to use their authorized devices.

youraccount_authorized_borrowers_none

• Does it mean other steam users can play my games?
• If so, do I get logged out of my games when they use them, or vice-versa?
• What are authorized devices? Something related to the list of allowed computers on SteamGuard? How do I add more devices?
• How do I authorize a user? Do I have to login on my steam account on their computers? DO they have to login on their steam account on my computer?
• Any more relevant considerations?

Authorized Devices is a component of Steam's upcoming Family Sharing feature. It is currently only available on the beta version of Steam to users selected for the Family Sharing beta test.

When you authorize a device, then you register that device and share your Steam library with it. (Not all games can be shared in this way.) You authorize a device by logging into Steam on the device and then authorizing it through the client via the Account settings. You may be asked to re-authorize a device via email if the hardware changes.

Shared libraries on authorized devices may be accessed by any users who log into Steam on that machine. Users borrowing from a library accumulate their own playtime and achievements.

If, at any point, the shared library is being used, then any other devices sharing that library will not have access to it. A library is in use when the owner or any user on an authorized device is playing a game using a license from that library. Borrowers cannot interfere with the owner or any other borrowers who are playing the game.

On the flip side, if you decide you want to play a game in your library, then whoever is currently sharing your library (if anyone is at all) will be given a few moments and then kicked off, unless they already own the game. They will be able to purchase the game in this window of time. If you own a game you will always play it from your own library, even if the device you are playing on is authorized to borrow other libraries that have the same game.

For more details, see the official Steam announcement.

It's the Steam Family Sharing program:

Steam Family Sharing allows family members and their guests to play one another's games (...) Once authorized, their library of Steam games becomes available for others on the machine to access, download and play.

This feature is currently (Dec 2013) in beta testing stage, available only for Steam users running the beta steam client (or so says Valve, I am running the Steam Client for Windows build Dec 3 2013, not a beta client), and who have joined the Steam Family Sharing Group (I did join the group).

How to share your game library:

First opt into the latest Steam Client Beta Update. Then, Family Sharing is enabled in one of two ways: You can either locally authorize a device to share via the Account tab in Settings, or respond via email to a user’s Steam request to share your previously installed games

Sharing doesn't work on a per-game basis, but your whole library (except some games such as "titles that require an additional third-party key, account, or subscription")

You can only share to 10 devices at a time, and a shared library may only be accessed by one user at a time. "If you decide to start playing when another user is already playing one of your games, he/she will be given a few minutes to either purchase the game or quit playing."

Borrowers will have access to DLC and in-game items, and are able to "purchase, trade, earn, or otherwise acquire in-game content", but these items go the the original owner's account.

Region restrictions "remain in place when lending or borrowing content."

If the borrowers commit abuse, cheat, fraud, etc, "your Family Sharing privileges may be revoked and your account may also be VAC banned"

(excerpts in quotes taken from the official page)