Billed as a "microblogging tool for classrooms", it seems more like a learning management system (or social network) that allows a teacher to create a group and hand out the group code to students, without an email address needed. Within Edmodo, teachers can create assignments, attach files, and students receive the notifications and can turn in assignments, too.
"Posterous is the dead simple way to put anything online using email. We love sharing thoughts, photos, audio, and files with our friends and family, but we didn't like how hard it was... so we made a better way. That's posterous. We're super excited to see what happens when blogging becomes as easy as email, and we hope you enjoy posterous as much as we do.