First of all for expert remarks about equalization see http://www.prorec.com/ where a search for Lionel Dumond finds some articles back in 1998 that are informative. I can only write very basic things like:

EQ is the most important digital effects tool the desktop audio engineer has.

Each instrument in a perfect mix should occupy their own band in the frequency spectrum.

No sound has one frequency. It is a mixture of frequencies. Almost all sounds contain "bass" frequencies ( < 200Hz) because (I assume) they don't require that much energy to generate.

Most of my poorly EQ'd mixes suffer from "bass build-up" that sounds dull or "blurry." High frequencies (used properly) give me the feeling of brightness and focus.

I can begin to identify sounds (like a good sound engineer) in terms of its frequency by looking at the five bands of my car stereo equalizer:

Deep bass is around 60Hz; bass is around 250Hz; the "middle" (where I think most of the annoying noise can be) is around 1000Hz; "treble" sounds begin at 3.5KHz; and the brightest stuff is about 10KHz.