The Wall of Death is usually as described above, but some variations do exist, the coolest of which is a sphere (mesh, or transparent, so you can still see the riders) which allows movement through all three dimensions.

This variation is particularly cool when you have three or so riders in the sphere, all going in different directions -- one going horizontally, one vertically, and one at perhaps 45 degrees to the others, or maybe horizontally in the opposite direction.

Generally, the people who do this are either adrenalin junkies, really extreme artists, or just plain nuts.