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.
Quilted cotton blended with polyester is often used as the shell material on saddle pads. The advantages of cotton are that it is light weight, is good at wicking sweat, and is available in many colors. Disadvanteges are that cotton does not wear particulary well.