Power Chords are used extensively in today's modern punk rock music. (Blink 182, NoFX, The Youth Ahead, A New Found Glory) Without getting into the theory of why a power chord sounds the way it does and diads and such, here is what a power chord is.
|(R+2)-|
|(R+2)-|
|-R----|
In this example, the root note is indicated by R, and then, on the two strings above it, you merely play two frets higher on the fretboard. The actual fretboard looks something like this.
 |--------------------|B
E|  |  |1 |  |  |  |  |R
A|  |o |  |o |4 |o |  |I
D|  |  |  |  |4 |  |  |D
 |--------------------|GE ;)
In this diagram, the fretboard is shown, and the numbers tell you what finger to place where.
  1. Index Finger
  2. Middle Finger
  3. Ring Finger
  4. Pinky Finger

The rhythm guitar part to Blink 182's Dammit is as follows.
e|---------|
B|---------|
G|-5-------|
D|-5-5-7-3-|
A|-3-5-7-3-|
E|---3-5-1-|
Where each chord is strummed in a punk pedal sort of way.
If any of this is wrong, just message me, im only a bassist so be gentle :P.