He was originally a host on some sort of sports show. He left that to be the host of the Daily Show, where he invented the Moment of Zen and the Five Questions, along with many other things that John Stewart tries to rip off.
Kilborn left the Daily Show to join CBS and host a show that appeared on television at roughly 4:52 AM. Because of this, only die-hard Kilborn-ites stayed up to watch.
Kilborn quit CBS because he was too good for that shit. He is now a big time pimp in films like Old School, Benchwarmers, and The Shaggy Dog, and has mad basketball skills.
On the April 21st, 2008 episode of TCR, Stephen revealed the shocking secret behind Kilborn's long absence from the Daily Show and non-existent career for the ensuing years: Stephen Colbert has in fact killed and cannibalistically eaten Craig Kilborn.