What is a "Power Chord"?
Power Chords (or "Rock Chords") are not truly chords at all, as they use only two notes of the scale - the root and fifth. Strictly, a chord must have at least 3 different notes! However, power chords are a great way of playing rock songs and are much easier to play than barre chords and are used extensively in rock music, especially Heavy Metal. They can be used for driving rhythm chords or for rock guitar riffs.
Power chords are also known as "5" or "5th" chords. So an E Power Chord is written as E5.
For more about Power Chords see http://powerchordtrainer.com
The Power Chord Trainer is a great easy was to start playing Power Chords!