Sure thing.
The Yahoo group is the traditional listserv. It's meant for general announcements and correspondence that the whole group should receive.
The Google group is a discussion forum, meant to be somewhat more active than the listserv. The forum is also meant to allow for more specialized or narrow conversation.
For the Yahoo listserv, all members receive all messages. On the Google discussion forum, each member can decide whether to join and also whether s/he will receive an email for each post to the group.
So, if a message should go to everyone, use Yahoo. If it's a more casual conversation starter, Google's the place.
If we were serving drinks, Google would be where the party's at. Whereas Yahoo is just business.
Incidentally, moving the listserv to Yahoo has taken a good deal of time, and I've just completed the transfer today. At the beginning of next, I'll generate a message to the group through Yahoo that will have this basic explanation in it, as our summer web movements have been many and potentially confusing.
-Justin
Justin D Burton, PhD
Web Editor, IASPM-US
Adjunct Music History Faculty
Rider University
Rutgers University
Montclair State University