Paper with a coating is smooth and shiny while uncoated paper is flat with little or no shine. Gloss stock makes colors look smoother, deeper, richer, with great color-contrast. Photo's and graphics tend to look better on gloss stock, while text heavy documents and artwork are often use matte stock.
Text is more easily read on paper with a matte finish. The softer looking dull surface of matte paper provides color contrast and clarity. Unlike glossy paper, matte stock is more forgiving of fingerprints, smudges and dust.
Uncoated paper is very absorbent, and ink dots will tend to spread outwards (i.e., dot gain), leading to a less precise and darker image than when printed on coated stock. A similar effect happens when a paper towel is placed on a spilled drop of coffee. The drop diameter increases and gets a ragged edge as the liquid spreads in the absorbent fibers. This dot gain can be minimized using sophisticated printing techniques, but it can't be eliminated. Coated paper is less absorbent and therefore dot gain is usually not an issue.