As a Steam account holder, you can now take advantage of Steam Guard, a new feature of Steam that enables an additional level of account security. Your account may already be protected by Steam Guard, as the new feature automatically begins protecting your account once you've verified your email address with Steam.
With Steam Guard enabled, anyone attempting to sign in as you from an unrecognized computer or browser must first provide additional, one-time authorization. A special access code will be sent to your contact email address, and this code must be entered into Steam before your first sign in on an unfamiliar computer is complete. You will also be notified if any sign in attempts from computers other than those you've authorized occur. Steam Guard essentially acts as a form of "User Rights Management," where you as the user have greater control over access to your stuff.