is the flexible built-in message reader in Emacs. It supports reading and composing both news and mail. It can also use web-based sources as inputs for its groups.
Gnus is a news and mail reader. It can display messages from a variety of sources, called "select methods", including:
- newsgroups from an NNTP server
- newsgroups from a local spool
- mail from a local directory (mbox, maildir etc supported)
- mail from an IMAP server
A stable version of Gnus is included in Emacs. Gnus is developed in a separate repository, which is regularly synced with Emacs.
The Gnus manual is accessible from Emacs' Info facility, but can also be read online.