Kerberos is a protocol for making authentication servers work. Microsoft's Active Directory uses Kerberos for all authentication purposes, so wherever there are Windows machines "in a domain" (as per Microsoft terminology, and as opposed to a "workgroup") then there is some Kerberos at work. A two-line summary of Kerberos is the following:
Every machine trusts the Kerberos server. The Kerberos server authenticates users, and give them tickets which they then show to the other machines. Tickets are limited in time and scope.
X.509 is a standard format for digital certificates, which bind identities to public keys. X.509 works when applied to a specific domain through a profile; the profile tries to narrow down the wide possibilities of X.509 into a workable set of options. The profile for "the Internet" is RFC 5280 and its most visible and widespread application is the issuance of certificates for SSL servers. Whenever you connect your browser to an
https:// Web site, then X.509 certificates are at work.
SET was another X.509 profile. It was rather thorough in that it was making X.509 much more simple, by removing hordes of useless options; and it also defined a few extensions which were generally needed. However, commercial success and scientific rationality have never been the same thing; SET never became a "de-facto standard" and is all but abandoned nowadays.
For teaching material, be sure to read the X.509 style guide from Peter Gutmann. Students love sarcasm and jokes.