aka fixies. Very fun to ride, I just finished building mine a while ago. Learning how to skid stop, my thighs are dominated. Pretty bruised :/ .
Best videos I could find:
http://www.youtube.com/watch?v=F0AFpq6jFok&feature=relatedhttp://www.youtube.com/watch?v=H63FivObTLMEdit for 100% more
definition: A fixed-gear bicycle or fixed wheel bicycle is a bicycle without the ability to coast. The sprocket is screwed directly on to the hub and there is no freewheel mechanism. A reverse-threaded lockring is usually fitted to prevent the sprocket from unscrewing. Whenever the rear wheel is turning, the pedals turn in the same direction. By resisting the rotation of the pedals, a rider can slow the bike to a stop,which will cause the tires to skid,without the aid of a brake. Stopping is sometimes known as "skidding" in the fixed gear world. A fixed gear bicycle can also be ridden in reverse.
However wikipedia is kinda wrong, skidding and resisting the pedals are waaaay different.