"Describing Folk Art" from Museum of American Folk Art Encyclopedia of Twentieth-Century American Folk Art and Artists by Chuck and Jan Rosenak:

The general term folk art encompasses a wide range of paintings, sculptures (from massive stone works to tiny fish decoys), and environments created by individuals who did not study art formally and systematically.

To describe particular types of folk art, a number of terms are commonly used. The reader should be aware, however, that exact definitions for thee terms tend to be controversial and the terms may overlap. In addition, while there may be general agreement about how to classify certain artists (e.g. Grandma Moses as a memory painter), collectors, galleries, and museums may not agree in every instance.

Environmental Art. Sculptural, often large-scale works that are usually constructed outdoors in gardens, yards, and public and private spaces. The artists commonly use recycled materials such as tin, glass, and found objects. Unlike other types of art, environmental works are created primarily for display and are generally not for sale. A well-known example is Simon Rodia's Watts Towers in Los Angeles.

Isolate Art. The highly personal work of artists who are isolated-physically or emotionally-from the cultural mainstream. The art is sometimes referred to as "raw" or or "primitive" in style.

Memory Painting. A style that depicts disappearing life-styles. The scenes are often peaceful ones of rural America before the advent of complex farm machinery. Among the best-known memory painters are Grandma Moses and Mattie Lou O'Kelly.

Naive or "Naif" Art. A direct, simple and unsophisticated style of expression. Such French artists as Henri Rousseau and Camille Bombois are often classified as naive or naif. While these terms are today used most commonly in connection with untrained European artists. American artists like Grandma Moses are sometimes referred to as naive artists. Professionally trained artists who have adopted the simplicity of the folk art style are sometimes called neo-naives.

Primitive Art. The art of primitive people in Africa, Asia, and Oceania. American folk artists generally do not like to be called primitive artists.

Visionary Art.Visionary Art. Art that is often religious in theme and is frequently based on dreams, visions, or voices. Religious messages may accompany a work. Among the better-known artists often termed visionary are Minnie Evans, Sister Gertrude Morgan, and Alex Maldonado. Visionary art is also used in a broader sense to denote works by folk artists with a unique vision.