Two-Factor Authentication is a type of multi-factor authentication. 2FA requires using two different factors from:

• Knowledge: an information subject kwows, such as password, answer on a special question etc.
• Possession: something subject has, which could be an specially created link, token, browser cookie and so on.
• Inherence: something subject embodies, including biometrics such as fingerprints, voice or face recognition, retinal scans, movement patterns.
• Location: information about place where the subject is physically.

An example of two-factor authentication is Google authentication. When a user logs in from a new device, in addition to authentication by username-password, he is asked to enter a six-digit confirmation code. The user can receive it via SMS, from a voice call to his phone, from the specially configured application etc.