Oil painting is a medium whose color pigments are suspended in an oil base. Though first used as a paint medium some 1200-1600 years ago, oil paints didn't reach a wide popularity until the height of the Renaissance by when it had virtually replaced tempura as the medium of choice of most artists.
Oil is a slow-drying medium allowing the artist a wide range of techniques to utilize and experiment with, as well as allows many additives to control translucency, sheen, drying time, density and body of paint, and other characteristics. Though oils are usually transfered by brushes, oil paint can also be painted well with palette knives, rags and sponges. Oil paints are typically dry to the touch in a day to a couple weeks, oils dry by oxidation as opposed to evaporation, thus may take six months to a year to dry enought to varnish. Art experts would cite 60-80 years for an oil painting to be considered completely dry.