Power chords are an extremely useful tool to have in your bag of tricks. Careers, if not entire genres of music have be based on them. As I said before these excellent little chords are composed of the root note and the fifth tone of the root scale. Since the third tone of the root scale is not present, power chords are not characterized as either major of minor.
In the following example, which is a I-IV-V change, I've added the octave above the root to the chords to add a bit of depth. This addition doesn't change the nature of the power chords, they're still just roots and fifths. The roots are E (the open sixth string) A (the open fifth string) and B (fifth string, second fret). Another aspect of these little beauties is that they lend themselves to tight rhythms and stacatto stabs due to their compact nature. Try to play this example evenly and tight. Start out slowly and work up to speed. (Leather jacket and low-slung guitar are optional.)