In order to learn power chords effectively, you'll need to really understand the names of the notes on the neck of the guitar. If you glossed over that page, you'll want to revisit it, and learn it well.
What a Power Chord Is
In some styles of music, particularly in rock and roll, it's not always necessary to play a big, full sounding chord. Often, especially on an electric guitar, it sounds best to play two-or-three note chords. This is when power chords come in handy.
Power chords have been popular since the birth of blues music, but when grunge music started to rise in popularity, many bands chose to use power chords almost exclusively, instead of more "traditional" chords. The power chords we are about to learn are "movable chords", meaning that, unlike the chords we've learned so far, we can move their position up or down the neck, to create different power chords.
Although the power chord pictured here contains three notes, the chord contains only two *different notes* - one note is doubled an octave higher. A power chord contains the "root note" - the root of a C power chord is "C" - and another note called the "fifth". For this reason, power chords are often referred to as "fifth chords" (written C5 or E5, etc).
The power chord does not contain the note which traditionally tells us whether a chord is major or minor. Thus, a power chord is neither major nor minor. It can be used in a situation where either a major or a minor chord is called for, however. Take a look at this example of a chord progression:
Cmajor - Aminor - Dminor - Gmajor
We could play the above progression with power chords, and we'd play it as follows:
C5 - A5 - D5 - G5
Power chords on the sixth stringTake a look at the diagram above - note that you do NOT play the third, second, and first strings. This is important - if any of these strings ring, the chord won't sound very good. You'll also notice that the note on the sixth string is circled in red. This is to denote that the note on the sixth string is the root of the chord. This means that, while playing the power chord, whatever note is being held down on the sixth string is the name of the power chord.
For example, if the power chord were being played starting on the fifth fret of the sixth string, it would be referred to as an "A power chord", since the note on the fifth fret of the sixth string is A. If the chord were played on the eighth fret, it would be a "C power chord". This is why it is important to know the names of the notes on the sixth string of the guitar.
Play the chord by placing your first finger on the sixth string of the guitar. Your third (ring) finger should be placed on the fifth string, two frets up from your first finger. Lastly, your fourth (pinky) finger goes on the fourth string, on the same fret as your third finger. Strum the three notes with your pick, making sure that all three notes ring clearly, and that all are of equal volume.