A forum is your website community on the Disqus network. When you register your website on Disqus, you are creating a forum with a unique shortname. Your shortname is different than your username.
Every website using Disqus has a unique forum which is moderated by their respective administrators. A forum consists of the comments and comment threads posted by other users. Users, a.k.a. community profiles, are not unique to forums since people can belong to any number of communities on Disqus.