Rather than having a single discussion area, with topics intermingling with other topics, themes of conversation are organized into a number of different containers, rather like the folder and file structure of your hard disk.
The top-level of organization is the category. Note that the categories here are different from the categories we have met in the other modules!
Within categories, the next level of organization is into forums. Forums consist of topics, and finally, users are able to creating postings on these topics. Thus categories, forums, and topics act like folders, with postings being analogous to the files, to continue the file system analogy.
Only forum administrators can create categories and forums. Topics (and obviously postings, since they are the real body of a discussion area) can be added by users of the forum. A topic is essentially a 'first' posting, with subsequent postings on that topic being replies to the topic subject.
Here is a diagram of the forums...