A listserv automatically distributes an e mail message from one member of a list to all other members on that list. Listservs maintain thousands of lists in the form of digests, electronic journals, discussion groups and the like. Some lists have 5 messages a day, some might have 20 or 100. Some lists only send out "publication" once a month, and are more for one way information, they are not really for discussion.
When you subscribe to a list, your name and e mail address is automatically added to the list. You will receive a standard letter of welcome (via e mail) telling you about the list. From that time on, you will receive all mail (postings) sent to the list by its members. You may follow the discussions or join in on them. If you respond, you can send your response to the list (in which case, all members of the list will receive it), or to an individual on the list. You can signoff (unsubscribe) from a list at any time.