Back in September of last year, Google announced a two-step sign-in system for Google Apps users, which combines a user's username and password with a separate verification code to make it harder for hackers to break into a user's account. Now, Google is making this option available to any Google account.
In a phased over the next few days, Google account holders will be given the choice to opt in for the two-step login by going to their account settings page and clicking on the "Using 2-step verification" link under the Security section inside Personal Settings. The image below shows where to go inside Account Settings to get started.
Once you've found the link, if the feature is ready for your account, a set-up wizard should help you with the following steps. The verification code can be sent to the user via a telephone call, SMS, or the user can create the code themselves using their Blackberry, Android or iPhone.
Once you've signed in with the code, it will prompt you for a new code every 30 days.
You can lean more about the aspects of this feature here.
Images from Official Google Blog