There are two main types of paper used in commercial printing: coated and uncoated
Coated Paper - a coated paper stock has a surface sealant and often contains clay. Coating papers reduce dot gain by restricting ink from absorbing into the surface of the paper. This sealant allows for crisper printing, particularly photos, gradients and fine detailed images. Coated stocks have numerous sheen options: gloss, matte, dull and satin finishes.
Gloss - a gloss coated paper has a high sheen, as those in a typical magazine. Gloss papers have less bulk and opacity and are less expensive than a dull & matte paper of equal thickness. A Cast Coated Paper (such as "Kromekote") has a very high gloss sheen made by pressing the paper against a polished hot metal drum while the coating is still wet.
Dull - a dull finish coated paper has a smooth surface paper that is low in gloss. Dull coated paper falls between matte and glossy paper.
Matte - a matte coated paper is a non-glossy, flat looking paper with very little sheen. Matte papers are more opaque, contain greater bulk, and are higher in cost.
Uncoated Paper - an uncoated paper stock has not been coated with clay or other surface sealants. Inks dry by absorbing into the paper. Uncoated papers comprise a vast number of paper types and are available in a variety of surfaces, both smooth and textured (such as "laid" and "linen"). Some fine quality uncoated sheets contain a watermark.
Weight - the weight of a paper refers to its thickness and is typically measured in pounds (such as 20#) and points (such as 10 PT). The higher the number, the thicker the paper for that "type" of paper. Paper weights in commercial printing can be very confusing. For example, a sheet of 20# bond (probably what you use on your copy machine) is about the same thickness as a sheet of 50# offset. A more meaningful measurement is to pay attention to is a paper's caliper. Case Paper Company has a downloadable chart as a PDF form that you may find useful to better understand the various paper weight terminology used in commercial printing.
Three general paper thickness categories used to describe the basis weight of matching stationery papers are writing, text and cover weight papers. They are commonly used for a company's matching letterhead, envelope, business cards and other collateral items.
Writing Paper - a letterhead-weight stock, typically 24# or 28# writing, and often has a watermark.
Text Paper - is thicker than a writing paper, but not as thick as a cover paper (card stock). Text-weight stationery paper is usually a 70# or 80# text.
Cover Paper - a card stock paper, such as those used for a business card or report cover. They are usually an 80# cover weight, although some brands of paper offer cover weight paper that is as thin as a 65# cover or as thick as a 100# cover or heavier.
Opacity - a paper's opacity is determined by its weight, ingredients and absorbency. A paper's opacity determines how much printing will show through on the reverse side of a sheet. Opacity is expressed in terms of it's percentage of reflection. Complete opacity is 100% and complete transparency is 0%.
Brightness - the brightness of a sheet of paper measures the percentage of blue light it reflects. The brightness of a piece of paper is typically expressed on a scale of 1 to 100, with 100 being the brightest. Most papers reflect 60-90% of light. The brightness of a paper affects readability, the perception of ink color and the contrast between light and dark hues.