I'll be interested if someone really knows this. I'm guessing that they weren't any very specific format, other than parts, but rather that they were sung by such groups as already described. I've several times seen in folksong books that a paticular song was once a glee.

A modern definition would be "anything sung by a glee club." These are likely to be called "chorales" now as in Roger Wagner Chorale or your high school.

On the other hand and drifting from the thread some, as to the nature of rounds, canons, fugues, etc. There is a truly fascinating discussion of these in Godel, Escher, Bach by DR Hofstadter. You might want to grab a quick read of that.