Posts made on forums within a course are sent as email copies to people who have subscribed to those forums. You may be subscribed by default to the forums in the courses you are participating in.
If you don't wish to receive the email copies, you can unsubscribe from any or all the forums. Just click the "unsubscribe from this forum" or "unsubscribe from all forums" link in any email copy of a forum post. You can also visit the forum and click "unsubscribe from this forum" in the "forum administration" block on the left.
If you unsubscribe from the forums, please make sure you visit your course regularly to check the forum posts, for there might be a lot of great discussions going on!
If there is a "news forum" in your course, you might not be able to unsubscribe from this forum because important announcements are made here by the course facilitator.