Teachers create an account for their classes and then pass along a joining "code" for parents. Parents can then create their own account and join various "groups". For example, a parent could join a middle school group and high school group.
Teachers decide to what extent they use Edmodo. Some may use it only as a communication tool to inform parents of projects and news going on in the classroom. Some may go as far as using the gradebook.
Students can put documents in a "backpack" and work on their schoolwork at home. Now, that is good news!
There are also Apps (many for free), a library, and social networking.