Fat tires, flat handlebars, etc. Mountain biking actually has many of it's own sub groups such as downhill, cross country, and dual slalom. I will not get into these, but if you want to know more, check out my links. Cyclocross Cyclocross is done during the winter, and is a combination of road and mountain biking, with some running thrown in. Every now and then, you have to jump off of your bike and jump over these little hurdles or run up very steep, sloppery hills.
Races go on a shortish circuit and do a few laps.
These races last around an hour on average. You probably know what BMX is, so I will not go into it Trials Trials is basically hopping around on a BMX looking bike. Recumbents Recumbents are a kind of bike that puts you close to the ground. You do not race on them, because they are illegal in races. You sit in a reclining position and they look plain strange.