The term "Self-Taught Art" simply refers to any object or set of objects in either two or three dimensions which was not initially intended to be art but was made for another purpose whether utilitarian or spiritual, whether public or private. The terms Folk Art and Art Brut are both subsets of this phenomenon.
In the United States, by the early 20th century, the term Folk Art began to be used to identify those items produced in earlier periods, made without the benefit of academy, which were perceived, by an audience to have become works of art based on aesthetic merit as perceived by that audience. That this work became considered Art was neither intentional nor understood by the person who made it. Sometimes, these objects initially marketed as craft such as weather vanes, trade-signs, duck decoys, whirligigs, or any hand made artifact of ordinary life and sometimes they were made for personal, decorative, commemorative purposes such as the portraiture of "limners," the private murals in homes or things like memory jugs or mourning pictures.
The primary impetus for the marketability of these items rests with the intense excitement created by some very rich and uncommon ladies (Abbey Aldrich Rockefeller as an example of the best known of the bunch) chasing after the detritis of the common folk in America as their parents were acquiring Impressionists from Europe.
Such momentum did this interest have that it spread rapidly, first amongst the wealthier folks whose interest eventuated in the spiraling of prices and the ever continuing interest in the acquisition of these things today whether in antiques shops or auctions.
By 1961, a Museum of American Folk Art was founded and only thirteen years later Twentieth-Century American Folk Art written by one of its founders, Herbert W. Hemphill Jr., was published.