An (electronic) mailing list is a distributor of mails to a large group of subscribers, one possible type of a digital forum. It's often used by software developers and administrators for communication among themselves and by organizations and corporations for newsletters (in one direction only).
I've recently subscribed to the CentOS mailing lists so that I can keep up to date with any new releases or updates. I'm subscribed to CentOS-announce. I get quite a lot of emails regarding bug fixes ...