A power chord is a two-note chord, with no major or minor quality to it. This is because power chords are just made up of the root and the fifth of the chord. The part that usually gives the chord a major or minor quality is left out of power chords.
For example, the power chord of C is made of only the notes C and G, the first and the fifth notes of the scale. As a result, the power chord is written as the name of the chord followed by the number 5. So the power chord of C is usually written as C5 on chord charts or tabs.
The great thing about power chords is once you have the shape down, you can just move them all around the fretboard.