Hi Lois, great question, thanks for asking! Here are the main differences in the forum types:
Discussion forums: Users can start new discussion threads or participate in ongoing threads.
Q&A forums: Users can post questions. Questions start out with a state of "unanswered" until they receive an answer. You can filter for unanswered questions. You can highlight the best community answers to questions, and mark answers provided by your support staff as "official answers".
Feedback forums: Users can post feedback and ideas, and vote for their favorite ideas. See which ideas are the most popular. Give updates to the community on ideas you are implementing. You can set the status of an idea to: In Review, Planned, Started, or Completed.
Article forums: These forums are mainly used for publishing content out to your community, including knowledgebase articles, announcements, or blog posts. Users can subscribe to the forum to receive the updates in email.
I hope that helps! Let us know if we can provide any additional clarification.