The MIT Kerberos Team produces three major products:

Kerberos Core Code
Also known as "Kerberos 5" or "krb5", this is the portable core source code of MIT Kerberos, which includes libraries and support programs. While historically this code was targeted at Unix, we currently strive to make the code as portable as possible.
Kerberos for Macintosh (KfM)
Kerberos for Macintosh now ships as part of the Mac OS X operating system.
Kerberos for Windows (KfW)
Kerberos for Windows provides a graphical user interface for managing Kerberos credentials on Microsoft Windows operating systems. It also provides Kerberos libraries for applications written against Kerberos APIs.

Generally, there are two series of releases: a current series and a maintenance series. Maintenance releases only get security vulnerability patches and critical bugfixes, while current releases get a wider variety of features and bugfixes.