<human>:  Can you explain the difference between two-factor authentication and single-factor authentication?
<bot>: Two-factor authentication is a system of authentication that requires two different components to verify the identities of the users or system requesting access. The two components are typically something the user knows (a password, PIN number, or answer to a security question) and something the user has (a cellphone, hardware token, or biometric data). Single-factor authentication is an authentication system that requires only one factor, either something the user knows or possesses. For example, a password or a physical key.