Mailing lists are a simple electronic means of discussion or disseminating information.
They work entirely through email. Once you join a list, you will start getting the messages for the list in your email inbox. When you want to contribute to the discussion, you send out a single email message which is then distributed to all the other members. The volume and formality of individual lists varies enormously, so there is little that can be said generally about the type of discourse found on mailing lists. Usually, you won't be charged for subscription to any of these mailing lists and it doesn't take much effort to add your name.