I have heard about the sport but never really understood what it was. So what exactly is it?

a road bike with a smaller version of a MTB knobby tire.

This should help...

http://www.usacycling.org/cx/

yeah!!! so for people whou looking on competition in cyclocross may bee uugly but not for racer..he hehh cyclocross racer loved snow, cold, rain, ranning whit bicycle on his back.....it's a short story about 50 minutes quick race on winter

http://www.cyclo-cross.com/intro.asp

gives a reasonable explaination

do you have to have a certain kind of bike or can you just put different tires on your regular road bike?