What is Two-Factor Authentication?
Two-Factor, also called "Two-Step" or "Multi-Factor" authentication, combines a "thing you know" like your username and password, with "a thing you have," often a confirmation message sent to your phone or other mobile device. Two-factor authentication is relatively easy to set up, and provides added security.
What Services Offer Two-Factor Authentication?
Google, Facebook, Dropbox, LastPass, Twitter, Amazon, Yahoo, and many more.