Yupo is synthetic, meaning, it is not made out of paper. It is smooth, and bright white, and really lets the colors "pop." Many artists like Yupo for watercolors because you can paint on layers of color. In addition, if you're not thrilled with what you've painted, you can wash the paint right off. The paper is non-absorbent and very strong. It also will lay flat even when wet.
You can see more of Carol Ann's artwork on her website here. She also teaches classes, so you can learn from the expert how to paint on Yupo.