Passwords - Basic Concepts

What are Passwords?
In a typical system, human users must authenticate themselves to a client through in general three possible ways:

1.Physical control of the device (“something you have”)
2.Presenting information, which is hopefully only in your own mind (“something you know”)
3.Authentication through unique biological attributes (“something you are”)

Combinations are possible and used for example in the ec/credit-card system. But for reasons of cost most systems take the second option. By that managing passwords is a real-world problem.