Emacs is a text editor from the GNU project. It's official page is
http://www.gnu.org/software/emacs/emacs.html and that'll tell you all about it.
Just recently I had a few troubles trying to install it with apt-get, and had to do it manually. Odds are that you don't have it installed on your computer. If you do, you should be able to open the program from a terminal just by typing "emacs". To exit use Ctrl-C then Ctrl-X. Use Ctrl-H then '?' to find general help menu, and you can lookup more specific subjects from there.
