Coated stock (like art paper or matte-coated) has a mineral coating baked onto the surface to smooth it out. Ink sits right on top, so colours print sharp and reflective — great for photos and saturated colour.
Uncoated stock has no coating, so the pulp fibres are exposed and you can feel — and see — a fine grain. Ink soaks slightly into the fibres, which mutes the colour compared to coated stock, but that same texture is exactly what reads as tactile and premium.
The same design can print with visibly different colour depending on the stock, so for anything that matters it's worth getting an actual proof on the real paper before committing.
When to use
Lean coated for photo-heavy catalogues and lookbooks; lean uncoated for business cards or invitations where touch and brand feel matter. Embossing and foil (→ emboss-deboss, foil-stamping) both read more dramatically on uncoated stock.