There are several different types.
If you are looking for something pretty, just to give a bit more support to a show saddle...the small velvet or leather ones although they have no padding, would do.
You can also get the thicker foam-filled naugahide or fuzzy ones, which offer a bit more support and padding. They are good for the usual mosey along the trail kind of driving, and the naugahide ones look good enough to show with...personally I don't like the coloured fuzzy ones used in a show ring, but don't mind them in black.
If you are interested in driving a bit more than just in a show ring, and expect to hit the trails hard...say working towards being competitive in the combined driving discipline, I strongly suggest a waffle pad set, paired with a harness with a good wide saddle.