Kerberos is a network authentication protocol for client/server applications. It allows authentication across a non-secure network in a secure way, using secret-key symmetric cryptography.
Kerberos uses security tokens called tickets. To use a secured service, you need to obtain a ticket from the Ticket Granting Service (TGS), which is a service running on a server on your network. After obtaining the ticket, you request a Service Ticket (ST) from an Authentication Service (AS), which is another service running on your network. You then use the ST to authenticate to the service you want to use. The TGS and the AS both run inside an enclosing service called the Key Distribution Center (KDC).
Kerberos is designed to be used in a client-server environment, and is rarely used in Web applications or thin client environments. However, many organizations already use a Kerberos system for desktop authentication, and prefer to reuse their existing system rather than create a second one for their Web Applications. Kerberos is an integral part of Microsoft Active Directory, and is also used in many Red Hat Enterprise Linux environments.