A mailing list is a software program that sits somewhere on a server. If someone then replies to a particular message, that reply will also generally be sent out to all subscribers of the list. Therefore, if you subscribed to the WebNet Mailing lists and then send a message to the list's email address, that message would be distributed to everyone who is subscribed to the WebNet list at the time. This way discussions can be held, somewhat similar to what happens on newsgroups.
Mailing lists exist for a variety of subject ranging from art to zoology and more.
Additional information on mailing lists and more mailing lists can be also found at the following sites.