In its most basic form, a mailing list is a distribution list that offers a means of communicating electronically with a group of people through a single e-mail address without having to know who all of the members of the group are and all of their individual addresses. Whenever someone sends an e-mail message to a mailing list address, that message automatically goes out to everyone subscribed to the list.
Mail lists allow people to join (subscribe) or leave (unsubscribe) them at any time. When you send a message to a mailing list, you address the email to the mailing list's name. When the mailing list server receives the message, it expands the name of the mailing list into each of the individual addresses currently on the list and it redirects your message to each of those people, called subscribers. Then, should the membership of the list change, there is only one file containing the addresses of the subscribers which must be updated.