As mentioned earlier, a group discussion site contains content, typically about a specific topic, that has been submitted by the members of the site. This makes such sites invaluable if you are looking for information on that particular subject, because the members of the site who contribute the content are likely to be either enthusiastic or knowledgeable about the subject.
These sites could be blogs, where the submitted content is in the form of blog posts and comments, or they could be bulletin boards where the content is in the form of threads of posts submitted by the members of the site.
