Though it's been available to Google Apps customers for a few months, two-step verification hasn't been accessible to regular Google users. That changes today.
Exactly what is two-step verification, you ask? Traditional username/password sytstems utilize a one-step verification system: you have to provide the system with something you know -- a valid username and password combination -- to gain access to the system. This works in many cases, but it can be easily exploited. If someone gets your credentials, they can log in from anywhere, and take control of your account.
Two-step verification, as the name implies, brings in a secondary factor; something you have. In this case, Google will require you to enter a passcode sent via SMS to the phone number associated with your account. This greatly reduces the chances of anyone gaining illicit access to your account, given that they'd need to possess not just your username and password, but also have physical access to your phone as they're trying to log in. It's a much better system, and offers a tremendous increase in security.
To activate 2-step verification, go to your Google Account Settings page, and in the Personal Settings section, you'll see a new link, "Using 2-step verification." Click that and follow the prompts to activate this powerful new security feature. Be warned, though; Google says it will take upwards of 15 minutes to properly reconfigure your account. That's a good chunk of time, but it's far less than what you'd have to spend recovering your data, should your account be compromised.
Note: If you find the idea of having to enter an SMS code every time you log in to Google to be just a little too much of a hassle, Google has given you the ability to have the system only prompt you to verify every 30 days.
