Paint is the general term for a family of products used to protect and add color to the surface of an object by applying a pigmented coating over it.
Paint is composed of a pigment, which provides color, mixed into a medium, which provides a liquid form for application. The combined liquid is applied to a surface using brushes or other methods. The medium contains a binder (usually a resinous material or polymer) which coats the pigment particles and eventually coalesces to form the dried paint film. The medium also often contains, or is modified with, a diluent (such as an organic solvent, or water) to make it more readily fluid.
The liquid paint eventually hardens, leaving the binder and pigment as a colored coating. Depending on the type of binder, this hardening may be a result of processes such as curing (in oil paint, this takes the form of oxidation of linseed oil to form linoxin), evaporation (most water-based paints are emulsions of solid binders in water; when the diluent evaporates, the molecules of the binder coalesce to form a solid film), cooling (encaustic, or wax, paints are liquid when warm, and harden upon cooling), etc.
Since the time of the renaisance, siccative (drying) oil paints, primarily linseed oil, have been the most commonly used kind of paints in fine art applications; oil paint is still common today. However, in the 20th centry, water-based paints, including watercolors and acrylic paints, became very popular with the development of latex and acrylic pigment suspensions. Milk paints (also called casein), where the medium is derived from milk, were popular in the 19th century and are still available today. Egg tempera (where the medium is egg yolk) is still in use as well, as are encaustic wax-based paints.
Paint is commonly used to protect the exterior surfaces of objects exposed to weather, such as houses and buildings, and has also been used for centuries in the creation of great works of art. Well known paintings include Leonardo da Vinci's Mona Lisa and Vincent Van Gogh's Starry Night.
Stain is another type of surface colorant, similar to paint. However, paint typically is intended to cover the outer surface of the underlying object, while stain is typically absorbed into the object and used used to bring out and accentuate the underlying surface.
Historical notes from Household Cyclopedia:
Ancient painted walls, to be seen at Dendara, Egypt, although exposed for many ages to the open air, still possess a perfect brilliancy of color, as vivid as when painted, perhaps 2000 years ago. The Egyptians mixed their colors with some gummy substance, and applied them detached from each other without any blending or mixture. They appeared to have used six colors, viz., white, black, blue, red, yellow, and green. They first covered the field entirely with white, upon which they traced the design in black, leaving out the lights of the ground color. They used minium for red, and generally of a dark tinge.
Pliny mentions some painted ceilings in his day in the town of Ardea, which had been executed at a date prior to the foundation of Rome. He expresses great surprise and admiration at their freshness, after the lapse of so many centuries.