Electronic mailing lists are a special usage of email that allows for widespread distribution of information to many Internet users. Software is installed on a server which processes incoming email messages, and, depending on their content, either acts on them internally or distributes the message to all users subscribed to the mailing list. Today, mailing lists are most often used for collaboration on various projects and as a way of distributing current news and other such information.
Sometimes these take the form of what is termed a "discussion list" or "listserv": 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, politics, scientific discussion, joke contests), even if the topic can range from extremely narrow to "whatever you think could interest us". 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.
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. While sometimes people see those restrictions as contrary to freedom of speech, this position is quite unrealistic: a different mailing list with more open rules can be started by anyone.
