Two-factor authentication is a security process in which the user provides two means of identification, one of which is typically a physical token, such as a card, and the other of which is typically something memorized, such as a security code. In this context, the two factors involved are sometimes spoken of as something you have and something you know. A common example of two-factor authentication is a bank card: the card itself is the physical item and the personal identification number (PIN) is the data that goes with it.
According to proponents, two-factor authentication could drastically reduce the incidence of online identity theft, phishing expeditions, and other online fraud, because the victim's password would no longer be enough to give a thief access to their information. Opponents argue (among other things) that, should a thief have access to your computer, he can boot up in safe mode, bypass the physical authentication processes, scan your system for all passwords and enter the data manually, thus -- at least in this situation -- making two-factor authentication no more secure than the use of a password alone.
Some security procedures now require three-factor authentication, which involves possession of a physical token and a password, used in conjunction with biometric data, such as fingerscanning or a voiceprint.