According to my Webster's New Collegiate Dictionary, Plain People are members of any of various Protestant groups (as Mennonites) esp. in the U.S. who wear distinctively plain clothes and adhere to a simple and traditional style of life excluding many conveniences of modern technology (as motorcars)
This pastel painting of an Amish woman is based on a painting I did last summer while visiting my parents in Vernon, New York. The subject is an Amish woman figurine. You can see the original painting here.