The manufacturer weighs a ream of 500 sheets of paper in its uncut state and gives it a number to indicate the paper’s weight. Weight indicates the quality (and usually the price) of the paper.
90 pound paper is a student grade and is rather thin. It buckles when wet and can’t endure much scrubbing for changes.
140 Pound paper is probably the most popular choice. It’s fairly stout, can be stretched to avoid buckling when wet, dries quickly, and is medium-priced.
300 Pound paper is like a board. It doesn’t require stretching, costs twice what 140-pound paper costs, and takes longer to dry.
Texture describes the surface finish on paper. The type of paper you choose gives you different effects with the paint. You may want a smooth paper for lots of detail or a textured surface to make sparkling reflections on water.
Hot press is even and smooth and makes a nice surface for prints and drawings. The paper has a slicker finish that you can use to create some interesting results. It’s more difficult to make soft transitions when using this paper, so you may have more hard edges than you want.
Cold press has a slightly bumpy texture. This is the most popular texture for watercolorists. The texture allows paint to settle into the texture pockets or sit on top and skip over the pockets, creating some different painting technique options.
Rough texture has an even bumpier surface than cold press. This surface is good for exaggerated rough texture techniques.
Papers can be purchased in a variety of ways.
Blocks are stacks of paper that are attached together. The sheets of paper are trimmed to some uniform size and then stacked upon each other. That stack of paper is then attached to a backing board with a padding glue. This glue is applied to all four sides of the paper. This is most commonly done to watercolor paper, to help the paper from buckling.
Rolls are a single sheet of paper that have been rolled into a cylinder shape. This allows the user to cut out as much of the paper desired in one dimension as the roll supplies. This can be presented for a variety of types of papers.
Sheet is a single sheet of paper. Large sheets of paper are often supplied in flat file cabinets to prevent damage, and may have a sheet of vellum between each sheet for additional protection. This can be presented for a variety of types of papers.
Packs are multiple loose sheets of paper encased in a plastic wrapping. This is particularly common for smaller sized papers.
Pads are thin stacks of paper that are bound together on one side. The stacks of papers can be bound either with a padding glue, coiled wire, or wire binding spine. The stack of paper has a backing board. This can be presented for a variety of types of papers.
Books are thick stacks of paper that are bound together on one side. The stacks of papers can be bound either with a padding glue or coiled wire. The stack of paper has a backing board. This can be presented for a variety of types of papers.