Watercolor pads are sized for wet techniques. An internal sizing holds the fibers in place and an external size prevents the quick absorption of water. A watercolor sheet will thus hold pigments on the surface of the paper and also allow a watercolor artist to work washes and techniques before the water absorbs prematurely inward.
A watercolor pad holds about a dozen (or more) sheets of watercolor paper.
Yupo is a non-absorbent polypropylene for experimental techniques.