Traditional music spoilt by words. - anon
A song is words (lyrics) set to music and may be accompanied by instruments or not. A song sung by one person is a solo, by two, a duet and so on with trio, quartet etc. Many voices together constitutes a choir if different parts are sung (i.e. there are arrangements for differently pitched voices) otherwise it is a mass solo.
The popular conception of Folk Songs is slow unaccompanied songs which go on for 35 verses. Some do but a lot don't. Another misconception is that they are all old and largely anonymous. Many are, but folk is a living tradition and there are many contemporary folk song writers producing new material.