What is a Two-factor authentication?

Two-factor authentication is a method of confirming users’ claimed identities by using a combination of two different factors: 1) something they know, 2) something they have, or 3) something they are. A good example of two-factor authentication is being frequently used on gmail.com. Every fresh login would ask for the password & a system generated one-time password sent on the registered mobile number or email-id.