Paraphrasing the website:

  Mailman is a mailing list manager (MLM); that is, software to help manage
  email discussion lists, much like Majordomo, LISTSERV, and the like.
  Unlike most similar products, Mailman gives each mailing list a web page
  and allows users to subscribe, unsubscribe, and change their preferences
  via the web.  Even a list manager can administer his or her list(s)
  entirely via the web.  Mailman integrates many common MLM features,
  including web-based archiving (though it also has hooks for external
  archivers), mail-to-news gateways, bounce handling, spam prevention,
  Majordomo-style email-based list administration, direct SMTP delivery (with
  fast bulk mailing), digest delivery, virtual domain support, and more.

Mailman is written mostly in Python (with a smattering of C where necessary
for security purposes), and includes hooks to make it easily scriptable and
extensible.  It is compatible with most web servers and browsers, and most
mail transfer agents (mail servers).  Mailman's documentation may be found on
its website.