Electronic mailing lists are a technique to use e-mail for widespread distribution of information to many Internet users. Software is installed on a server which processes incoming e-mail messages, and, depending on their content, either acts on them internally or distributes the message to all users subscribed to the mailing list.
Sometimes these take the form of what is termed a "discussion list": a subscriber uses the mailing list to send messages to all the other subscribers, who may answer in similar fashion. Thus actual discussion and information exchanges can happen. Mailing lists of this type are usually topic-oriented (for example, Bondage, BDSM, Femdom etc.). In this they are similar to Usenet newsgroups, and share the same aversion to off-topic messages. The term discussion group encompasses both these types of lists and newsgroups.
Free web-based services offering an easy way to run and maintain such lists were popular in the late 1990s, but many of these were taken over or went bust, so that the only popular provider is now Yahoo! Groups. The new version of Google Groups includes free mailing list services as well as access to Usenet.
Some mailing lists are open to anyone who want to join them, while others require an approval from the list owner before one can join. In even stricter mailing lists, every message must be approved by a moderator before being sent to the rest of the subscribers. Moderator approval is usually employed to keep a high average quality of posts and weed out spam.