Folk songs are literally songs of the "folk:" the indigenous people of a region. Folk songs may be traditional, originating in a distant place (or time) and adapted, revised and renewed through something called the "folk process." Many American folk songs originate in the British Isles, or even northern Europe, brought over by generations of immigrants.
In the 1950s and 1960s American folk songs were brought into the mainstream of American music by popular folk singers and bands like Pete Seeger, The Kingston Trio, Peter, Paul and Mary and Joan Baez. These musicians continued the folk process and the tradition of folk-music as political commentary, a American tradition going back to the time when "Yankee Doodle" became a popular protest song in the days of British rule of the American colonies.
Not all folk songs are of ancient origin; folk singers like Seeger (Turn, Turn, Turn), Woody Guthrie ("This Land is Your Land") and others wrote original songs that have, over the course of the last 50 years endured and have, themselves, been adapated. And now properly labeled "folk songs."