The best water color paper is made from 100% cotton. Paper with low cotton content is useful for practice and for paintings with low amounts of water.
Watercolor paper is made from cotton mixed with wood pulp. Typical cotton contents are 100%, 50%, 25%, and 0%. More cotton is better.
Why is paper with a high cotton content good for watercolor painting? Paper made from cotton is stronger than wood pulp paper. It is more stable, and will not deteriorate as badly with time. Cotton is more absorbent than pulp; think of a cotton shirt vs. a plank of wood: which one will be more absorbent?
Cotton can handle multiple layers of pigment, and can withstand color lifting and the various watercolor techniques without damaging the paper. Too much water or too many layers will cause wood pulp paper to pill. The paper starts to break down, and you will see little balls of paper fiber moving across the surface of the paper as you paint. It seems like the paper is simply dissolving.
Wood pulp paper is good for practicing, learning brush strokes, and learning color mixes. It is okay if you are not going to use a lot of water (like line and wash) or if the painting does not have multiple layers.