Paper with a coating is smooth and shiny while uncoated paper has a rougher texture and flat finish with little or no shine. Gloss stock makes colors look smoother, deeper, richer, with better color-contrast than uncoated stock.
Photos and graphics also tend to look better on gloss stock, while text heavy documents and artwork are typically more suitable for matte stock.
Text is easier to 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, dust, and smudging.
Uncoated paper is very absorbent, and ink dots will tend to spread outwards (i.e., dot gain).
This leads to a darker image that doesn’t reproduce as precisely as it does on coated paper.
It’s similar to what happens when a paper towel is placed on a drop of liquid. The droplet increases in diameter and the edges become ragged.
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 less of an issue.