Its the latest "next big thing" in exercise programs. Its a combination of compound movements, body weight exercises, & aerobic exercises usually done in as short a time as possible. The Gym Jones guys use a variation of it, and they're the group that did the training of the actors and stuntmen for 300.
What is Crossift?
http://www.crossfit.com/cf-info/what-crossfit.html
Two articles about 300 (warning - lots of attitude in these articles, and the rest on the Gym Jones site):
http://www.gymjones.com/knowledge.php?id=35
http://www.gymjones.com/knowledge.php?id=36