An email mailing list is like sitting around with a group of friends discussing a given topic. Or, similar to a discussion board except the exchange of messages takes place via email.
Messages and replies are sent directly to your email address - you do not need to go to a special page or board to view them.
Most mailing lists are topical in nature. For example there may be an email list exclusively for the discussion of Pokemon, or one for nothing but horses. There are email lists for just about every topic imaginable.
The replies to the messages are sent directly to the list for all members to read. This makes for an orderly, fast convenient way to discuss your favorite topics.
If you already have email it is easy to join a mailing list about your favorite topics. The next article explains the different types of email lists out there.