Biometric modalities describe types of measurement. The modalities are broken into two categories: physiological and behavioral. Physiological biometrics are things like the image of an iris, face or the pattern of the ridges on the fingers. Behavioral biometrics are things like a signature, the way an individual types or walks. The most common biometric modality is the fingerprint, which has actually been used for identification since ancient Babylon.
Other common biometrics are the iris (colored part of the eye) and face. These three constitute the vast majority of biometrics used in practice. Other biometrics include voice, how a person walks, retina, hand geometry, ear shape, heartbeat, typing style and even smell. It turns out that humans are full of unique things. This is the result of random variations in genetics and the womb. Even identical twins have unique biometrics!