Emacs is a text processing environment. Everything is a command: even hitting the down arrow is calling a specific command in Emacs Lisp. This means you can modify almost any behaviour or Emacs. Want to add some new action to the down arrow? You can easily do that with Emacs.

(I'm not trying to convert you, I'm just trying to get across what Emacs is)

Emacs exposes lots of rich functionality to the user.