Guitar power chords are special types of chords that sound tight and powerful with the distorted guitar, where regular chords such as: major, minor, seventh and etc... create too much clutter.
Compare the A minor chord
to the A power chord
Power chords are mostly used in Metal, Trash, Alt-Rock and other heavy styles.
In a nutshell they only consist of the root note and the fifth. The root note is the base note that lays in the name of the chord.
For example in the C power chord the root note is the note of C. In order to find the fifth, count from the root note five steps higher: 1-C, 2-D, 3-E, 4-F, 5-G. The G is the fifth.
Here is the location of the C power chord on the fretboard.
In order to play the C power chord press the A string at the 3rd fret and the D string at the 5th fret with your fretting hand, and stroke these strings with your pick.
You can learn how to reduce the noise that is created when you accidentally touch the nearby string with the pick here.
In guitar song books the power chords are marked as a figure 5 added to the root note, so the C power chord indicates as C5, the A power chord as A5, and etc...
Very often in order to create a more massive sound, the 3rd note is added to a power chord, it's actually the same root note again but located an octave higher.
I found it is more comfortable to hold the index finger as when you play a bar chord, but you really need to press only one string with the index finger that has the lower root note.The fifth and the higher root note you can play with the ring and the pinky finger.
One more things about power chords is that they are neither major nor minor chords, and here is one interesting twist I found while practicing:
If you take any song or part of a song that is played on an acoustic or clean electric guitar, for example let's take the VERSE of Fade to Black song by Metallica, the actual chords for clean guitar are:
A minor → C major → G major → E minor
Straightforward convert them into power chords like this:
and play it with the distorted electric guitar, using any rhythmic pattern you feel fit, you can have a heavy metal remix for the acoustic theme ;)