Your voiceprint unlocks the door of your house. Your retinal scan lets you into the corporate offices. Your thumbprint logs you on to your computer. Unfortunately, the reality of biometrics isn't that simple. Biometrics is the oldest form of identification.
Dogs have distinctive barks.
Humans recognize each other's faces. On the telephone, your voice identifies you as the person on the line. On a paper contract, your signature identifies you as the person who signed it. Your photograph identifies you as the person who owns a particular passport. What makes biometrics useful for many of these applications is that biometric measurements can be stored in a database.