Gnus uses the url library to fetch web pages and Emacs/W3 (or those alternatives) to display web pages. Emacs/W3 is documented in its own manual, but there are some things that may be more relevant for Gnus users.
For instance, a common question is how to make Emacs/W3 follow
links using the browse-url functions (which will
call some external web browser like Netscape). Here's one
way:
(eval-after-load "w3"
'(progn
(fset 'w3-fetch-orig (symbol-function 'w3-fetch))
(defun w3-fetch (&optional url target)
(interactive (list (w3-read-url-with-default)))
(if (eq major-mode 'gnus-article-mode)
(browse-url url)
(w3-fetch-orig url target)))))
Put that in your .emacs
file, and hitting links in W3-rendered HTML in
the Gnus article buffers will use browse-url to
follow the link.