its all done in XML
very good if you know what youre doing.
i recommend installing notepad++
from the sound of things it seems as though you are interested in making some very heavy modifications. i would advise to look at some sample mods that others have posted online so you can look at their notes. EQinterface is a great resource, though some of the classic mods are no longer hosted.
there are some user-created ui mods here in forums, but you will have to search. start
here.