This is a very good page on how to use the equalizer built into iTunes.  The equalizer basically determines how much bass and trebble is added to each song and at what frequency.  Basically, using these adjustments is a really quick and easy way to make your music sound much, much better.

Read the article, or play a song, and try sliding the different bars up and down.  You’ll get the hang of it.  But the articles easier.

http://www.methodshop.com/gadgets/tutorials/ituneseq/index.shtml