Here's a great article I just found which describes the differences between tubular and clincher tires.
Clincher tires are found on the majority of bicycles. Clincher tires consist of a tire and a separate inner tube. Tubulars are glued directly onto the wheel rim, and can be a bit time consuming to install, along with being much more expensive. Only a few of my really serious racing friends use tubular tires.
Lynn Smythe is a self-confessed bike addict and founder of the Bike Diva blog. When she's not out riding her bike with a local cycling club, Lynn can be found participating in a variety of charity athletic events to help raise money for organizations such as the Leukemia & Lymphoma Society and the Multiple Sclerosis Society . In her free time, Lynn likes to participate in time trial bike races and duathlons. She also writes a blog at www.bikediva.net.