When the term ~/.gnus.el is used it just means your Gnus
configuration file. You might as well call it ~/.gnus or
specify another name.
Back End
In Gnus terminology a back end is a virtual server, a layer
between core Gnus and the real NNTP-, POP3-, IMAP- or
whatever-server which offers Gnus a standardized interface to
functions like "get message", "get Headers" etc.
Emacs
When the term Emacs is used in this FAQ, it means either
GNU Emacs or XEmacs.
Message
In this FAQ message means a either a mail or a posting to a
Usenet Newsgroup or to some other fancy back end, no matter of
which kind it is.
MUA
MUA is an acronym for Mail User Agent, it's the program you
use to read and write e-mails.
NUA
NUA is an acronym for News User Agent, it's the program you
use to read and write Usenet news.