Discussion lists allow the students and instructors in a course offering to communicate with each other whenever they want, no matter where they live or work.  A student in New York may ask a question at midnight, and the instructor could answer that in a discussion list at 8:00 am the next morning from her office.  That way, everyone in the class can access the dialogue whenever it is convenient for them. 

Discussion lists are essentially threads of consciousness where one person can reply to another person and so on until that particular thought is fully played out for the purposes of benefitting the participants.