An equalizer, or EQ, is a stereo component that controls the frequency response of your music. Frequencies are divided up into bands, or frequency ranges and each band is controlled by a slider on the EQ. Generally, the higher the number of bands, the better quality the equalizer is. A graphic EQ usually has some type of graphic representation of the frequency levels for each band as the music plays.
The idea is that every room has different a different type of resonance, so a graphic EQ can be used to shape the frequency response of the music. That sounds like a great idea. The problem? The more control you have, the easier it is to mess up your sound!
For instance, everyone likes thumping bass response so the temptation is to crank up the very low frequency bands. Well, the 50 hz band or the 60 hz band really only control frequencies that are so low they are barely audible to the human ear. Basically those frequencies sound like a low rumble. Cranking those bands up can actually make your mix sound muddy. The thump or punch to a music mix is actually at about 100 hz. Try leaving the entire line of bands flat, but then slide 100 hz up a ways. You will hear the difference immediately.