A shaped (or "numnah") saddle pad is saddle shaped, and is designed to be used alone, or with a square saddle pad. A shaped / numnah pad should only be slightly visible below the saddle.
Neoprene is a synthetic, closed cell rubber. Neoprene saddle pads are good at distributing pressure, and at preventing sliping and clinging to the horse's back. Disadvanteges of neoprene are that it does not wick moisture, and can trap heat.