Miller is the skill associated with the milling labor. Millers mill plants at either a quern or millstone, placing the resulting goods into bags.
Milling produces one unit of dye, flour or sugar per item milled as well as 1 or 2 seeds. Milled items have no extra quality associated with them.
A quern is powered by the dwarf himself, while a millstone uses outside mechanical power (either a water wheel or a windmill). Using a millstone is obviously quicker, though it is also more difficult to set up.