Computer software, such as Listserv or Majordomo, distributes the messages to everyone on the list. Any replies go to the administrator only and not to the whole list. Moderated - list members can send messages to the moderator for redistribution to all subscribers. Open - any message sent by a list member is automatically received by every member of the list, including both original posts and replies. Announcements and moderated lists are often used for disseminating medical information, research, and news.
Open lists are common for groups whose main goal is to support members and encourage communication.
Be aware that unmoderated discussion lists tend to be much higher in volume. The positive side of both newsgroups and unmoderated mailing lists is that they encourage an open exchange of ideas and dialogue. The down side, of course, is that some of what you read may not be things you need or want to hear. Once you choose a list or lists you'd like to join, you have to know the correct commands in order to subscribe. Each type of list software has a different format for these commands, but instructions are always provided.