According to Apple's own documentation (
here &
here) the Grouping field in iTunes is supposed to link tracks, with the same value, within an album so that they are treated as a unit when shuffling in "Shuffle by Grouping" mode. The iPod doesn't have a corresponding mode so I'm not sure how useful this feature is in practice.
It is worth noting that the iTunes Grouping field maps to Sub-Genre or Mood in other applications. Personally, I use the field to store keywords which I then use to build smart playlists. iTunes 9.x has the option to expose Grouping in the Column Browser alongside Genre, Artist, Album and Composer (also new).
Comments on the other hand are just that. I use them occasionaly to record information about a track that I don't want in the track title.
tt2