eldoc mode as the name suggests provides documentation for Elisp files. This is a very useful and cool mode and shows the function signature in the mode-line when your cursor is on a particular function. It also provides info on the variables. Check out the screen-shots below.
To start eldoc mode, add the following to your .emacs file
;;Turn on documentation in elisp mode (add-hook 'emacs-lisp-mode-hook '(lambda () (turn-on-eldoc-mode)))
To add c-eldoc mode, get the latest c-eldoc.el, copy it in your load-path and then add then following to your .emacs file.
(add-hook 'c-mode-hook 'c-turn-on-eldoc-mode)