First, lets start with what 'two-factor authentication' is. It involves two steps to logging in.  With one step, you just needed your username and password.  You entered the info and you're logged in; this is one step.  In two-factor authenticataion (or two-step authentication) uses more than just your username and password. For example, one of the first two-step authentication methods I came across was with my Discover Card account.