Paper with a gloss finish is shiny (it reflects a lot of light) while matte paper is flat and dull in appearance with little shine or light reflection.
Gloss stock makes colors look smoother, deeper, richer, with great color-contrast. Photos and graphics tend to look better on gloss stock, while text heavy documents and artwork generally are more visually appealing on matte papers.
Text is also easier to read on a matte finish. The softer dull surface of matte paper offers good color contrast and clarity.