Power chords are usually just the lowest 2 or 3 notes of a regular chord. The main advantage to em is they don't have any specific major or minor quality to em, so you can use them over just about anything. Another advantage is when using distortion, you aren't using the higher strings which tend to sound like crap in full chords. Lastly, they're punchy in the bass end and leave 'space' for other instruments or vocalists in the mix.