RE: Basics of Parametric EQ
peak - boosts or cuts the center frequency.
shelf - boosts or cuts the frequencies above (for a high shelf) or below (for a low shelf) the center frequency.
high/low pass - completely rolls off frequencies above (for a low-pass) or below (for a high-pass) the center frequency.
depending on the EQ you may not have a high/low pass. some EQs have set types at each band, for example: low shelf, peak, peak, high shelf. this is because those would be the most common setup for EQing a single instrument.
some EQ's like sonitus allow you to do anything you want with each band. but all the band really is, is a center frequency. i.e., you want to boost the bass guitar line, so you use one band at 400hz (400hz being the center frequency) and you add 2dB of gain to make it louder.
the "quality" knob or Q is it is generally referred to, determines how "wide" the band is going to be. for example, with a peak EQ, a Q of 1.4 is about one octave (if i remember right) and a Q of 0.7 is two octaves, although it's not on a scale that easily translates. generally for "fixing" things a narrower EQ is better (2.0 or higher), a Q of 1.0-1.4 or so is good for adjusting specific tonalities of an instrument, and 0.7 is good for subtle changes. none of this is set in stone, you have to hear it, these are just starting points that may or may not work at all.
for shelf and low/high-pass EQs, the Q determines how sharply the curve will be set. a Q above 1.4 will actually start to boost or cut the center frequency in the opposite direction of the applied gain (so for a low/high pass, it will boost since those types of EQ always cut). if you want a sharper rolloff without the boost, combine a shelf with a high/low-pass EQ.
gain, as i said, is a function of how much louder or softer you want this particular frequency (or range of frequencies, in the case of a shelf) to be. gain has no effect on high/low-pass EQ, which always cut down to silence.
the best thing you can do to learn what an EQ does is to make big changes and start training your ear to hear smaller and smaller changes. also you should check out Voxengo's SPAN plugin, which when you hold down the ctrl key will act like a big boost for whatever frequency and you can sweep it around to hear the different tonalities of instruments at specific frequencies, and hear the way ranges of frequencies sound in general.
hope that helps.