Harder to ride than a unicycle, but easier than a BC wheel, an Ultimate Wheel is a unicycle with no seat post, nor, indeed, a seat. Its basic construction is a bike wheel (usually of 26 inches or larger diameter) with a solid disk of wood instead of spokes. The pedals are screwed into metal inserts in the wooden disk.
Most unicyclists can do ultimate wheel after a few hours practise, but it's important to polish the tyre sides (SIDES ONLY!) with silicone polish, so that it slides easily over the sides of the legs. The quick-start method is to ride as if you're unicycling, but turn left slightly when the right pedal is at the top, and vice versa - this produces a wobbly forward motion, and keeps the wheel away from your legs.
Riding the ultimate wheel is more tiring and far more impressive than basic unicycling, although the scope for tricks is reduced.
To get on an ultimate wheel, either stand holding the wheel with one hand, get one foot on, then hop up and go... or for the more flashy, roll the wheel away in front of you and jump on!