Watercolor paper is divided into three categories according to the surface of the paper: rough, hot-pressed (HP), and cold-pressed (NOT).
As you'd expect from the name, rough watercolor paper has the most textured surface, or most prominent tooth. It's sometimes described as having a pebbly surface, a series of irregular rounded shapes like a pebble beach. On rough paper the paint from very watery washes tends to collect in the indentations in the paper, creating a grainy effect when the paint dries. Alternately, if you whisk a dry brush lightly across the surfaces, you'll apply paint only to part of the paper, the tops of the ridges and not in the indentations. Rough paper is generally not regarded as a good paper for painting fine detail, but is excellent for a loose, expressive style of painting.
Hot-pressed watercolor paper has smooth surface with almost no tooth. It's smooth surface is idea for painting fine detail and for even washes of color. Beginners sometimes have problems with the paint sliding around on the smooth surface.
Cold-pressed watercolor paper is sometimes called NOT paper (as in not hot pressed). It's the paper in between rough and hot-pressed paper, having a slightly textured surface. Cold-pressed is the most commonly used watercolor paper surface as it allows for a good amount of detail while also having some texture to it.
Soft-pressed watercolor paper is in between hot-pressed and cold-pressed, with a slight tooth. It tends to be very absorbent, sucking in paint, making it harder to paint dark or intense colors.
Once again it's important to remember that surfaces vary from manufacturer to manufacturer. The watercolor papers shown in the photo above are all classified as rough.
