Kerberos is a network authentication protocol. It is 
designed to provide strong authentication for client/server 
applications by using secret-key cryptography. A free 
implementation of this protocol is available from the 
Massachusetts Institute of Technology. Kerberos is 
available in many commercial products as well.

Version 5.0

When you log in, your client contacts the Kerberos server 
and uses your password to prove your identity. In return it 
receives a special "ticket granting ticket" (TGT), valid 
for several hours.

When your client wants to contact a network service on your 
behalf (e.g. to mount a network file share), it first 
contacts the Kerberos server with the TGT and the name of 
the service. The Kerberos server issues a service ticket, 
which your client presents to the actual network service. 
The network service validates the ticket (without 
contacting the Kerberos server) and allows the connection.

kerberos is an authentication protocol as posted earlier
the version of kerberos is 5.0 in windows and in prewindows 
authentication protocol is NTLM. ntlm is new technology lan 
manager as authentication protocol.