Coated paper is often misunderstood to mean “glossy”. While a coated paper can be glossy, paper can also be coated in semi-glossy and matte finishes. Coated paper is simply a paper with an agent added to the surface. The purpose of the agent is to fill in the tiny pores and spaces between the fibers of the paper. When these little spaces are filled it helps to improve certain print qualities such as the brightness, sharpness and clarity.
Uncoated paper is paper without any kind of agent applied to the surface. An uncoated paper is best used on jobs that do not require an exceptional finish and perhaps print qualities such as brightness, sharpness and the clarity of images are less important.