via http://education.zdnet.com/?p=2417 "Like Twitter, it’s a microblogging site, but it builds in significant additional functionality to support classroom interactions." "When you create an account, you designate yourself as a teacher or student. Teachers can create groups that students join when they create their accounts (students can join multiple groups and teachers can create and/or join multiple groups); when a group is created, the site generates a group code that must be entered to join. Then, messages, files, links, and assignments can be sent to whole groups."
G. Biegel, and V. Cahill. Proceedings of the Second IEEE International Conference on Pervasive Computing and Communications (PerCom'04), page 361--. Washington, DC, USA, IEEE Computer Society, (2004)