Authentication refers to the process of confirming identity. While often used interchangeably with authorization, authentication represents a fundamentally different function.
In authentication, a user or application proves they are who they say they are by providing valid credentials for verification. Authentication is often proved through a username and password, sometimes combined with other elements called factors, which fall into three categories: what you know, what you have, or what you are.
Single-Factor Authentication relies on a password. Example: a school website that only requires validating a password against a username.
Two-Factor Authentication relies on a piece of confidential information in addition to a username and password. Example: a banking website that validates a password against a username and then requires the user to enter a PIN known to only the user.
Multi-Factor Authentication (MFA) uses two or more security factors from independent categories. Example: a hospital system that requires a username and password, a security code received on the user's smartphone, and fingerprint.