In the (default) Standard forum for general use, students will see the description above the list of discussion threads. Students will be able to start new discussion threads.
In a Single simple discussion the instructor posts a question and students are only able to reply. They cannot start a discussion topic of their own.
In a Q&A forum the teacher starts a discussion topic with a question or prompt and students respond to it. By default, a Q&A forum requires students to respond before being allowed to see one another's responses--great for urging independent thought!
The Standard forum displayed like a blog works much like the default forum style. However, the first post of each discussion is displayed like a blog entry so that users can read it and then respond by clicking the "Discuss this topic" button below the post.