A discussion list is one where every subscriber can post messages to the entire list. Only subscribers can post - messages coming from addresses which are not subscribed will be refused.
In addition to posting, a discussion list supports methods for subscribers to join and leave the list by themselves, typically via a form on the owner's web site(s). It also keeps the list clean by automatically removing addresses which bounce repeatedly. For our discussion lists, addresses which bounce on three successive days are removed after the third bounce. More here: BounceHandling