Although some are closed to the public, many thousands are open to anyone. This means your words of wisdom can reach several thousand people each time you post a message. If all you do is publicize your product or services, you probably won't like the results you get. On the other hand, if you offer information, advice, or participate in ongoing discussions, you may find that your business increases. Each list is unique.
Subjects vary as do participants and discussions.
However, there are some qualities shared by all lists. Each list will have a particular tone and local culture. Additionally, each list is similar to a community. It is made up of individuals who communicate to the larger group via email. You join a list by subscribing, either directly for open lists or indirectly through a list owner for private lists.