Even if you know you should use two-factor authentication whenever possible, you may be aware that there is also "two-step" authentication. Over at Lifehacker, they explain the difference between both types of extra security methods.

Basically, two-step authentication (2SV / two-step verification) includes any service that make use of, for example, Google Authenticator or SMS codes as additional methods to access to an account. While, on the other side, two-factor authentication (2FA) is any service that requires at least two authentication mechanisms, and where two of those mechanisms fall under different categories with respect to: knowledge factors ("something you know"), possession factors ("something you have"), and inherence factors ("something you are"). Smart cards and USB keys with the FIDO technology would fall into the two-factor authentication mechanisms.

Lifechaker, GrahamCluley