Power chords are three-string chords which use can use to play almost any rock or heavy metal song you can think of. You usually write them down as fives, like E5 that you play like this:
The template for a power chord is 1-5-8 (as usual, the numbers are referring to the tones of the major scale for the key you are using). If you just use classic power chords the sound can be a bit cliché and make all your own riffs sound the same. But as luck would have it, there are a lot of variations such as the four (1-4-8) and the nine (1-5-9).
Naren at Metalguitarist.org has made a simple video where he goes through a number of variations of power chords and demonstrates what they sound like: