Technically User is not correct regarding if it is the wheel or hub, ad a special hubed wheel is needed, yet that pretty much means the whole wheel since it is almost always faster and cheeper to just by a different wheel when the hub is not what you need than it is to completly re-spoke a wheel hub. And that hub is the difference, sort of. If you try to single speed a new road bike with verticle drops, HERE IS THE DIFFERENCE: a single speed is just getting rid of the shifting capability and keeping a little chain tension, but you can keep the same hub. But trying to put on a fixed hub wheel is a nightmare, because the reverse tension is extremely hard on the tensioner, if it is a spring or wheel hanger single speed kit, thus in these fixie conversions the solution is to add a horizontal drop kit of some kind or even mod the frame.