Power chords are movable guitar chords that can be played up and down the neck of the guitar at any fret.
Power chords use the 1st and 5th notes of a major scale, and are also referred to as 5th chords. The root note is typically repeated an octave up. Power chords do not use a 3rd note from the giving these chords a different sound than major or minor chords.
The pictures and guitar chord diagrams below show how to play power chords on each group of strings.