Also knowns as a Refrain meaning repeat. Chorus songs form by the far the majority of Folk Songs. The Chorus is the common part sung between verses of different lyrics.
The chorus is often joined in (by encouragement) by any audience and some songs are expected to have a mass chorus (e.g. Wild Rover and the No! Nay! Never! refrain).
Sometimes a very small part of the Chorus changes each time to catch the unwary, and sometimes it echoes the last line of the preceeding verse.