I was going to do a simple overview of EQ for a tutorial (and still might one day), but the folks over at AudioTuts+ have already done it.  And it’s a great tutorial:

Knowing how to use an equalizer is a fundamental skill for anyone working with audio, yet it is one of the most abused. Here are some tips and tricks for using your EQ more effectively. You’ll notice there are more don’ts than dos on this list; that’s because EQ is best when used in moderation.

via 8 Easy Steps To Better EQ.

I love the AudioTuts+ site, and you should check it out.  They also have one of the best looking sites on the web.  I have learned plenty from them, mostly when it comes to Logic Pro.

The best tip in all of it is to cut EQ more often then your boost.  Adding in an EQ boost can also add in noise, as you are elevating a level.  This is especially true when you are working with an analog EQ (like on a mixer), but applies to digital and DAW EQ as well.

Again, all props to AudioTuts+, a great site.