Looking for a way to keep your online accounts (cloud storage, banking, social media, email, etc.) as secure as possible?
Two-factor verification (also known as two-step authentication, abbreviated to TFA) is a process involving two stages to verify the identity of a user trying to access services in a computer or in a network. This is a special case of a multi-factor authentication which might involve only one of the three authentication factors (a knowledge factor, a possession factor, and an inherence factor) for both steps. If each step involves a different authentication factor then the two-step authentication is additionally two-factor authentication. (Wikipedia)
For example, to access any of my WordPress sites as an administrator, I have to provide something I know (my password) and demonstrate that I possess something bound to the account. In most cases, the latter is a security code that Google Authenticator (on my smartphone) displays and that changes every 60 seconds, whereas for other accounts it may be a six-digit code sent via SMS to me.
For many applications, you can save a computer or device for a period of time without having to authenticate with both factors. This makes logging into frequently-used accounts easier, albeit not quite as secure.
For a look at which services offer two-factor authentication, visit http://twofactorauth.org.