Biometrics literally means "life measurement." Biometrics is the science and technology of measuring and statistically analyzing biological data. In information technology, biometrics usually refers to technologies for measuring and analyzing human body characteristics such as fingerprints, eye retinas and irises, voice patterns, facial patterns, and hand measurements, especially for authenticating someone. Often seen in science-fiction action adventure movies, face pattern matchers and body scanners may emerge as replacements for computer passwords So, Biometric systems can be defined as "automated methods of verifying or recognizing the identity of a living person based on a physiological or behavior characteristic".
Automated methods By this we mean that the analysis of the data is done by a computer with little or no human intervention. Traditional fingerprint matching and showing your drivers license or other forms of photo ID when proving your identity are examples of such systems.
Verification and recognition This sets forth the two principal applications of biometric systems. Verification is where the user lays claim to an identity and the system decides whether they are who they say they are. It's analogous to a challenge/response protocol; the system challenges the user to prove their identity, and they respond by providing the biometric to do so. Recognition is where the user presents the biometric, and the system scans a database and determines the identity of the user automatically.
Living person This points out the need to prevent attacks where copy of the biometric of an authorized user is presented. Biometric systems should also prevent unauthorized users from gaining access when they are in possession of the body part of an authorized user necessary for the measurement.
Physiological and behavioral characteristics This defines the two main classes of biometrics. Physiological characteristics are physical traits, like fingerprint or retina that are direct parts of the body. Behavioral characteristics are those that are based upon what we do, such as voiceprint and typing patterns. While physiological traits are usually more stabile than behavioral traits, systems using them are typically more intrusive and more expensive to implement