The most common is watercolor paper, but other surfaces such as vellum, parchment, clay mineral panels, sumi rice paper, or thin fabrics such as silk can be used.
Watercolor paper generally has to be stretched before use. This is especially true of lighter weight paper, which will otherwise buckle after absorbing water. Watercolor painting boards are helpful tools for stretching your paper. They provide a hard surface on which to mount the paper in preparation for painting. After wetting and then stretching the paper, allow it to completely dry before painting to prevent it from rippling.
You can paint directly onto paper without stretching it, but it would be better to try this using a heavy paper that can absorb a fairly large amount of water without wrinkling.
Stronger and more enduring, professional-grade papers are acid- and lignin-free and made of cotton fiber rather than cellulose. With proper treatment, a painting on high-quality professional paper can last hundreds of years. The paper's texture and surface is brought out by the transparency of watercolor paints — one of the desirable qualities of a fine watercolor painting.
Watercolor paper is specially made to be resilient and to absorb water evenly and slowly. Because watercolors are transparent, the surface takes on enhanced importance. Watercolor paper comes in many weights in smooth (hot press) and textured (cold press) surfaces. The type and amount of sizing in the paper controls water absorption and paint flow.
Hot press watercolor paper has a smooth surface, while cold press watercolor paper has a slightly raised, bumpy surface. Rough watercolor paper has a surface that has even more texture. A "not" watercolor sheet is “not hot press,” and the term is often used instead of cold press. The decision comes down to which surface you prefer for the type of watercolor painting you practice. Sizing is often added to make a watercolor paper more water-resistant, keep it from absorbing too much moisture or pigment, and maintain the brilliance of watercolor paints or inks. Internal sizing is added while the paper pulp is still in a liquid state, while external sizing is applied to the surface of the paper after the sheet is formed and dried. Some papers are both internally and surface-sized.