Bluegrass is a relatively new form of folk music, with direct roots in the much older tradition of American old-time music, which in turn has roots in the Irish traditional music and Scottish traditional music of immigrants from the Britain and Ireland (particularly the Scots-Irish immigrants of Appalachia). Bluegrass songs are played with each melody instrument switching off playing the melody each time through, while the others revert to backup; this is in contrast to old-time music, in which all instruments play the melody together, when indeed they are playing together rather than solo.
The bluegrass style was invented in the first half of the 20th century by Bill Monroe and more specifically by Bill Monroe and His Bluegrass Boys, formed 1939; the bluegrass sound wasn't defined until Monroe's 1945-47 band. Unlike mainstream country music, bluegrass today still tends rely on the traditional stringed instruments (usually fiddle, banjo, acoustic guitar, mandolin, and upright bass).
Movies about bluegrass or featuring bluegrass themes: