Emacs is the everything program. It is designed to be a text editor.
However, it is so much more than a simple text editor.
Emacs can: code, play games, web browse, turn into a terminal shell, and even turn into your Window Manager!
Like Richard Stallman said, "People can do all their computing inside of emacs without ever leaving Emacs."
You can even code features into Emacs using a special variant of Lisp called Emacs Lisp (or EL).