In the Standard forum for general use, the default forum type, both students and teachers can start a new discussion or thread. Both teachers and students can also reply to any existing thread at any time.
In A single simple discussion the teachers posts a question and students are able to reply-only. They cannot start a new topic of discussion. This is useful if you wish to keep a discussion focused.
The Question and Answer forum is best used when you have a particular question that you wish to have answered. The teacher posts a question and students respond with possible answers. By default a Q and A forum requires students to post once before viewing other students' postings.
The Standard forum displayed in a blog-like format works like the standard forum for general use, but the first post of each discussion is displayed (as in a blog) so that users can read it and then choose to respond by clicking the "Discuss this topic" button bottom right of the post. Once you click "Discuss this topic" you'll be given the option to "Reply."
Each person posts one discussion. This one is pretty self explanatory. Everyone can post only one discussion topic. Replies are unrestricted. Everyone can reply to any discussion topic as many times as they want. See the teacher's view of an "Each person posts one discussion" forum below.
For more information regarding the forum activity, see: https://docs.moodle.org/38/en/Forum_activity