If the post office worked the way listserver programs do, you would give the post office a list of your friends' addresses. You then would send to the post office the letter that you wanted distributed to your friends. The post office would photocopy it, address each individual letter, then mail each of the letters. What's more, if a friend were to write back, replying to the letter you sent, the post office would distribute that person's reply to you and all your other friends. That's basically how listservs work.
Lists make it easy for a group of people to have discussions via e-mail.
Instead of each of you having to send the mail message to each of the people who want to have the discussion, you send the message to an e-mail address that identifies the list. Reading a list, then, is a matter of reading your e-mail. Once set up, the listserv can distribute e-mail with very little human intervention. Having a listserv list might at first sound confusing and perhaps daunting, but it's quite simple once you sit down and actually send e-mail to one to get a feel for how it works. In addition, owners can moderate lists and receive various error messages related to them.