the midi files are for listening to the score to hear what it
sounds like before you download the postscript.
the mpeg files are here in case the midi support on your sound
card is not all it could be; you might hear something better
from the mpeg.
the printable versions of the scores are in postscript format. most
operating system discributions come with a postscript interpreter and
previewer; the one i use is gv. if you use a system like
microsoft windows or a mac, you'll have to get the software you
need yourself from http://www.cs.wisc.edu/~ghost/.
the pdf versions can also be printed, though i'm not as sure
about them as i am about the postscript versions. :) if you
have a pdf viewer set up as a plugin, you can also enjoy viewing
the pdf version of the score right in your browser.
the png images are here so you can preview what the score looks
like before you download the postscript. they're not meant to
look good printed.
the lilypond source files are here so you can edit the score as
you wish, to change fingerings, for example. check out the
lilypond homepage
for an explanation of mudela, the language in which lilypond
source files are written.
one caveat to consider: lilypond is under continuous development, and
isn't capable of engraving everything i need for all these scores.
you should check the comments in the mudela source of each to see
where things are missing.