Two-factor authentication (2FA) is a security system that requires two distinct forms of identification in order to access something. Two-factor authentication can be used to strengthen the security of an online account, a smartphone, or even a door. 2FA does this by requiring two types of information from the user. These factors are knowledge, possession or biometric.
A knowledge factor is something that you know. The most common example of this is a password. In this form of authentication knowledge of a secret distinguishes you from all other individuals. And the authentication system simply needs to check to see if the person claiming to be you knows the secret.
Possession Factor
A possession factor is something that you have. This could be a smartcard or a smartphone. This object must be with you anytime that you want to authenticate. Although this factor is not full proof, it is more secure then knowledge based alone.
Authentication based on "something you are" will employ behavioral and physiological characteristics of the principal. These characteristics must be easily measured accurately and preferably are things that are difficult to spoof. For example, we might use are a retinal scan, fingerprint reader or a voice print.