Sometimes called "announcement lists", a newsletter list is one where the owner posts messages and all subscribers simply read those messages.
When you visit a web site and are encouraged to "subscribe to our newsletter", that is an example of a newsletter list.
"The owner" may in fact sometimes be two, three or more people, each of whom knows how to post messages to the list. See NewsletterPosts?.
In addition to posting, a newsletter 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 newsletter lists, addresses which bounce on three successive mailings are removed after the third bounce. More here: BounceHandling