A discussion mailing list is much like a bulletin board, forum or chat group, except that it's conducted by e-mail. Messages posted to the mailing list are seen by all other members of the list. One may choose to receive all messages sent to the list, or they may choose to receive a single message which contains the entire text contained in all the messages sent in a day. The latter practice is called digest form. Mailing lists are a great deal of fun, and a valuable resource in the exchange of information.