Kerberos is a networked authentication system developed at the Massachusetts Institute of Technology., A central trusted "ticket-granting server" provides "tickets" which allow other machines to authenticate each other. Granting of specific rights, called credentialing, to authenticated machines can be by a separate secure server.
Specifications and documentation
The separation of credentialing from authentication is not part of all authentication systems, but offers the ability to separate the administration of those two functions, which is a check-and-balance for personnel security of administators.
There was considerable controversy  over Microsoft's usage. Basically, they took the Open Source tool, added "enhancements" that made their version incompatible with everyone else's, and tried to impose licensing conditions on their code that would have made it very difficult for anyone to write a compatible version. You could not even see the specification without signing a non-disclosure agreement.