Good posting by jldiaz.

To start editing a file you will also need to use a text editor of some kind. 'vi' comes pre-installed. I myself only know the most basic commands so I use it only for simple stuff (like updating the already mentioned configuration files). Still, it's a powerful editor and can be used for writing e.g. python programs, if you want to.

Here's a guide to using vi: (there are others, easy to find with google):
http://www.cs.rit.edu/~cslab/vi.html

UPDATE: I forgot to mention that the 'esc' key (essential with vi) is the physical button with that curved little arrow.