An euqlizer splits the sound spectrum form 20hz to 20khz up into strips. Each strip can have it's volume set differently. That lets you tinker with the sound of some instruments. Like drums are in the 200-400 Hz range so you can push or reduce them fairly easily. String instruments are in the 1000's of Hz range.
Voice is a problem. depending on the singer it can be anywhere from Bass (100's) to treble (10,000). It depends on who you are adjusting and if a particular instrument is at the same frequency. If an instrument (like those crappy electronic drums) is drowning out a good vocal you can reduce them by cutting down the level on the lower bands of the equalize.
Unless you can strip the vocal track out of the music mix there is not a whole lot you can do to fix a bad mix.