The .NET Developer's Guide to Windows Security

Chapter 6. What Is Authentication?

Authentication answers the question, Who are you? When Alice logs in to a machine, the machine challenges her to prove her identity by asking for a password (or something else, like a smartcard). This is one example of authentication. Another is when Alice is already logged in to a machine and requests a file from another machine via a file share. Even though she's already logged in to the local machine, that doesn't help the remote machine at all. The remote machine wants proof that this is really a request from Alice as opposed to some attacker on the network just pretending to be her. Kerberos (Item 59) is an example of a network authentication protocol that protects most Windows systems.

It sometimes helps ...