The traits used are of two types:
- Based on the body (physiological traits). These include fingerprints, face recognition, scent, voice recognition, iris (eye), or retina recognition.
- Based on the behaviour of a person.
Most often, the two types are combined, as in voice or signature recognition. Most of these biometrics are seen as "behavioral", but some are linked to the physiology, e.g. of the vocal tract, or of the hand.
A person may be identified by a fingerprint, or by letting the person sign a document. The data could then be kept. Someone who is checking can again take the fingerprint or the signature, and compare it to the old version.
When selecting features, those choosing must also look at social acceptance. Taking a fingerprint may be ok, taking a blood test may not be. Some biometric data is also easier to fake, e.g. signatures.
Other than for passports, biometric methods are also used in many banks.