Biometrics can cover a wide range of methods. The two main categories are physiological biometrics and behavioral biometrics. The former is more common, and refers to unique physical characteristics, such as facial recognition, fingerprints or finger geometry, retina or iris scan, and finger/hand veins. A type of behavioral biometrics would be keystroke dynamics, which measures the time it takes to press each key, delays between keys, characters typed per minute, and so on.