Steam Guard
In March 2001 Valve introduced Steam Guard functionality to protect users against hijacking of accounts.

Steam Guard

Steam Guard takes advantage of the identity protection provided with Intel's second generation Core processors and compatible motherboard hardware to allow the user to lock their account to a specific computer.

With Steam Guard enabled, anyone attempting to login as you from an unrecognized computer or browser must first provide additional, one-time authorization.

Steam Guard is an additional level of security that can be applied to your Steam account. The first level of security on your account is your login credentials: your Steam account name and password. With Steam Guard, a second level of security is applied to your account, making it harder for your Steam account to fall into the wrong hands.

When Steam Guard is enabled on your account, anyone attempting to login to your Steam account from an unrecognized computer must provide additional authorization. A special access code will be sent to your contact email address, and this code must be entered into Steam before your login is complete. Once you've verified your email address with Steam, Steam Guard becomes available for your use.

Once enabled, you will be required to access your email and provide the special access code from Steam Support when logging into Steam from any computer which Steam doesn't recognize.

What do I need to do to take advantage of this additional layer of account security?

1. Update Steam. Steam should automatically update itself but you can force it to update by clicking "Check for Steam Client Updates"
2. Be sure your contact email address is verified with Steam. You must verify your contact email address with Steam. You can check whether your email address is already verified by visiting Steam Account Settings. A verified address will be marked as "Verified." To verify your email address, follow these instructions.
3. Restart Steam

That's it! With Steam Guard enabled, you will be asked to enter the special access code sent to your email address each time you login to Steam from an unrecognized computer.

Is there a limit to the number of machines that can be authorized at once?

No, there's no limit. Steam Guard is aimed to protect the value that is yours, not limit your access to your stuff. As always, you can access your Steam account and library from as many machines as you'd like.

How do I get a new access code?

Exit Steam, restart your computer, and log back into your account. This will generate a new verification email.
Ensure that you enter the most recent code sent to you - older codes will not work.

What if I do not receive a Steam Guard email when I am prompted for the access code?

Please check to make sure that you are accessing the email address that is registered to your Steam account.

If you do not receive the e-mail at all, check and make sure it is not being filtered as spam. Please try adding "support@steampowered.com" to your contacts or trusted senders list within your email client and request a new access code.

Even though Steam instantly sends an email, you may encounter a delay with some email providers depending on their server load and processing times. Contact Steam support if you have not received the verification email after 3 hours.