A single simple discussion - is just a single topic, all on one page. Useful for short, focussed discussions.
Standard forum for general use - is an open forum where any one can start a new topic at any time. This is the best general-purpose forum.
Each person posts one discussion - Each person can post exactly one new discussion topic (everyone can reply to them though). This is useful when you want each student to start a discussion about, say, their reflections on the week's topic, and everyone else responds to these.
Q And A Forum - The Q & A forum requires students to post their perspectives before viewing other students' postings. After the initial posting, students can view and respond to others' postings. This feature allows equal initial posting opportunity among all students, thus encouraging original and independent thinking.