The Discussions tool contains Forums and Topics. A Forum is a container for one or more Topics where the actual conversation takes place. Students post their original threads and reply to others' threads at the Topic level.
You can create one Forum for all Topics in the class or use Forums to organize Topics into categories. For example, a Forum named "Questions about this Class" could contain one Topic where students can post general questions. Another Forum named "Weekly Discussions" could contain multiple Topics that correspond to a weekly discussion subject.