It is a combination of software that allows you to distribute email to a large number of people that are all subscribers to the same mailing list.
Somebody sends an email message to the list address.
The software picks up the message from the list address and sends it to the members of the list. Click on the link to the left and review how a mailing list works. Manual Jump ahead in the presentation to find out what it would be like to operate the mailing list all by yourself, by hand. Automatic Jump ahead in the presentation to see how much easier it is to let the computer handle the mundane task of receiving mail and sending it out to everyone on the mailing list. The two big options are either receiving individual messages or a digest.