Steam Guard is a security measure designed to help better protect every Steam account. With Steam Guard, each time a new computer logs into an account, an email is sent with a verification code. Use the verification code on the new computer, and access to the Steam account is unlocked.
To prove the system's effectiveness, Valve co-founder Gabe Newell, publicly announced his Steam password during the GDC in San Francisco two weeks ago.
Steam Guard is provided through a free automatic update to the Steam client.