An Online Forum (also referred to as 'listserver') is an e-mail server that exchanges messages between persons on a subscription list. All the persons on the list exchange mail simultaneously. When a user sends a message to the list server everyone on the list receives a copy. Likewise when someone responds to the listserv, all the people on the list will see the response. To the user, the Online Forums appears simply as regular e-mail.

Online Forums are generally question and answer forums that allow groups of people with specific knowledge and interests to share ideas over long distances with the simplicity of sending e-mail. To join a listserv usually requires nothing more than sending an e-mail message to the listserv operator. Unsubscribing can be done the same way. Some listservs provide a browseable archive or digest versions that can be mailed periodically if the user wants to limit their incoming e-mail load.

This list divides Pollution Prevention (P2) listserv information into 4 categories: