Power chords
A power chord is just the root and the fifth, often with the root doubled an octave up. With no third it is neither major nor minor, which is why it sounds so solid with distortion.
Power chords in every key
One shape, every chord
Put your index finger on the root (low E or A string) and your ring and pinky fingers two frets higher on the next two strings. Move the same shape up or down and the chord takes the name of the root note.
Keeping them tight
Let the side of your index finger lightly touch the higher strings to mute them, so only the two or three notes of the power chord sound when you strum.
Questions
Why is it written as a 5?
The 5 means the chord contains only the root and the fifth. E5, for example, is E and B.
Can I use power chords in place of normal chords?
Usually yes. A power chord fits wherever a major or minor chord with the same root is played, which is why rock players use them so much.