Watercolour paper can be hand-made or machine-made. The former has an irregular surface texture and is usually of the highest quality – but that comes at a premium price. Most papers are machine-made, and then the surface texture is more regular. How regular and how pronounced varies from make to make – some papers have a highly regular, deep texture so that the surface is almost “patterned”. I prefer something more subtle.
Watercolour paper can be made of rag or wood pulp. The former is more expensive. Some is cellulose based which I find most unsatisfactory for any sort of quality work. Paper comes with different amounts of internal and external sizing and in 3 surfaces: Hot Pressed, which is fairly smooth and can be difficult to work; Cold Pressed (normally called NOT) which is medium textured; and rough – which is self explanatory.
Paper also comes in different weights. The heavier the paper the less likely it is to buckle when wet and the more ‘abuse’ it can take.
90lb is just about alright for sketching, with a pen and applying very light washes, but for anything else most artists prefer 140lb or heavier. 200lb is a pleasure to work on, and 300lb is almost like stiff card.