Paper

Watercolor paper is designed to properly support the paint and be sturdy enough to withstand the painting action. Painting on thin paper such as used for computer printers, for example, is possible but the paper will buckle (if not dissolve entirely), the paint will remain on the surface, running off in an uncontrolled manner.
The surface of watercolor paper can vary in its smoothness from very smooth to very rough. A watercolor painting on rough paper will result in quite a different look than a similar painting on smoother paper. The artist selects paper with a finish to give the desired effect.
Rough-surfaced paper is called "Rough", a smoother surface but still slightly rough is called "Cold Press" (aka "Not"), the smoothest surface is "Hot Press."
Watercolor paper is typically made of 100% cotton rag. Papers of lesser quality might be mixtures of rag (cotton) and other materials. Some artists use only better grade papers to get the effects they desire. The quality of paper can make a significant difference in the result.
The thickness of commercial watercolor paper varies from 90 pounds to 300 pounds (the weight of a ream of about 500 sheets of the paper.) A middle weight, common thickness, is 140 pound paper. While almost any paper of any thickness will buckle when wet, paper less than 140 pound thickness will buckle severely from the water and should be "stretched" before using. Even 140 pound paper will show some stress when wet watercolor is applied over large areas. 300 pound weight paper does not buckle severely and does not have to be stretched.
When using individual sheets, one might consider "stretching" the paper to prevent severe moisture-based buckling of the paper. Using this method, the paper is soaked in a tub of cool water. The amount of time to soak will depend on the weight of the paper (fifteen minutes maximum for the heaviest sheet). Once the paper is at its full saturation, lay the sheet on a drawing board and use gummed tape or staples to secure the paper to the board. Be sure all air bubbles are pressed out before affixing to the board. Lay the board flat and let dry before the painting is begun. Any drawing can be done before the soaking. The term "stretching" is actually a little deceiving since the paper is in reality, "shrinking", tension applied to the paper as it dries, much like a drum head.
A watercolor "block" is a pad of paper bound on all four sides, which prevents buckling in a similar way as stretching. There are usually 20 sheets of paper in a block. After the top sheet is used, it is peeled off and the next sheet is exposed.
Besides commercial watercolor paper, arches paper is also commonly used and preferred.