Buckskin is a color of horses; it also refers to other things that are the color of a buckskin horse, such as the color of some breeds of dogs. The horse has a tan or gold colored coat with black points (mane, tail, and lower legs).
Buckskin occurs as a result of the cream dilution gene acting on a bay horse. Therefore, a buckskin has the Extension, or "black base coat" (E) gene, the agouti (A) gene (see bay for more on the agouti gene), which restricts the black base coat to the points, and one copy of the cream gene, which lightens the red/brown color of the coat to a tan/gold.
Buckskins should not be confused with dun-colored horses, which have another type of dilution gene, not the cream gene. Duns always have primitive markings (shoulder blade stripes, dorsal stripe, zebra stripes on legs, webbing). Unlike buckskins, who have the creme gene, dun horses have the dun gene. However, it is possible for a horse to carry both dilution genes; these are called "buckskin duns" or sometimes "dunskins."
Also, bay horses without any dun gene may have a faint dorsal stripe, which sometimes is darkened in a buckskin without a dun gene being present. Additional primitive striping beyond just a dorsal stripe is a sure sign of the dun gene.
A buckskin horse can occur in any number of different breeds, though at least one parent must be from a breed that carries the dilution gene, and not all breeds do. Since 1963, the American Buckskin Registry Association has been keeping track of horses with this unique coat color.