2-Step Verification offers a strong extra layer of protection for Google Accounts. Once enabled, you’re asked for a verification code from your phone in addition to your password, to prove that it’s really you signing in from an unfamiliar device. Hackers usually work from afar, so this second factor makes it much harder for a hacker who has your password to access your account, since they don’t have your phone.

Today we’re adding even stronger protection for particularly security-sensitive individuals. Security Key is a physical USB second factor that only works after verifying the login site is truly a Google website, not a fake site pretending to be Google. Rather than typing a code, just insert Security Key into your computer’s USB port and tap it when prompted in Chrome. When you sign into your Google Account using Chrome and Security Key, you can be sure that the cryptographic signature cannot be phished.

More: http://googleonlinesecurity.blogspot.com/2014/10/strengthening-2-step-verification-with.html