A discussion list is simply a list of e-mail addresses of people interested in the same subject.
Someone sends out a message and it is distributed via email to everybody on the list.
Members can reply to the messages, send new messages, or just lurk, reading the messages without participating. Each time any list member posts a reply, it's distributed via e-mail to everyone else on the list. The two most frequently used MLMs are Listserv and Majordomo. A reply is sent that gives all the details one needs to know about the list and how to participate. Soon, one begins receiving mail from other list members.