OK. The bike wheels that you know are known as clincher wheels (the tire has a rim bead that hooks up and CLINCHES to the rim when under pressure).
For years, pros raced only on tubular wheels. Tubular tires are essentially complete tori, a selaed donut of tire and tube. To install them on rims, you would glue them. There is no rim bead.
This is Mavic's Reflex tubular rim. Note the difference between rims you are used to seeing.
In cross, running tubular tires means you don't have to worry about pinch flats (there is no rim lip to pinch the tube on). This means you can run lower pressures to get more traction while not worrying about snake bites (yes, ok, some tubulars can get pinch flats, it CAN happen but it is far less likely).
All the top Euro pros race on tubies, many of them Dugast handmade tires. yum. Check out the tire selection at www.cyclocrossworld.com
to see more.