Technically, a refrain is any line or series of lines that’s repeated at least once throughout the course of a song. A chorus section is just one particularly common form of refrain, but certainly not the only kind.
A song or poem can have more than one refrain, as in Willie Nelson’s “Reasons to Quit,” which repeats the title during the verses but also has a separate and totally different chorus section.
The folk song “Black is the Color of My True Love’s Hair” bookends each verse with one refrain in the first line and a different but related refrain in the last line.
Poetic forms that make heavy use of refrains include the pantoum, villanelle, and sestina. The circle-back is an effective but less common type of refrain in songwriting.
No matter what kind of refrain a songwriter is using in their latest song-in-progress, it always helps to have a reason for that repetition. The verse section can be used to enter the refrain from different angles, changing its context and meaning. Lyricists skilled in this kind of clever, productive repetition gravitate–consciously or not–toward refrain lines that contain double-entendre and other ambiguous words that can change readily according to context.