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I have heard about power chords being used in a lot of metal and similar music but I have no idea what they are.

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So called 'power chords' are the result of using only 1 and 5 of a chord in a key. They are neither major nor minor, lacking the defining 3 from the key. As with an E5, E and B (and optional higher E) are played.

They work well with distorted guitar sounds, as their harmonics (emphasised by the overdriven sound) don't clash too much. Putting in a 3rd produces other harmonics, which do clash.

On guitar they're easy to play, with just 2 or 3 fingers, and with drop D tuning, even easier.

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    FWIW I would say power chords are slightly harder to play in drop D when playing 3 strings (double 1st + 5th) because you have to bar, but easier when playing 2. They're both easy, though.
    – user428517
    Jun 27, 2018 at 14:37
  • @LCIII power chords by themselves are not really more powerful than full triads. They are more powerful than single notes. Mostly, the power that the name is alluding to comes from the distortion that's used, in Killing In The Name and most other hard rock/metal stuff. Distortion works well on single notes and, as Tim said, also on power chords because the notes in a power chord have a frequency relation of almost exactly 2:3. ...
    – leftaroundabout
    Jun 27, 2018 at 15:00
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    ...Major chords have conceptually also a pretty simple frequency relation of 4:5:6, but unfortunately we're stuck with the 12edo tuning system which approximates major thirds not very well. The detuning is small enough that most people don't consciously notice it in a clean sound, but distortion blows up the difference frequency to very incoherent intermodulation across the whole audible spectrum. That's the reason triads aren't much used in metal, and power chords are perceived as more powerful.
    – leftaroundabout
    Jun 27, 2018 at 15:00
  • @sgroves: Barring many strings with the index isn't hard if one isn't having to do anything else as well. What makes barre chords difficult is the need to hold a barre while reaching with other fingers.
    – supercat
    Jun 27, 2018 at 15:38
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    @supercat Well, everyone is different. Barring 3 strings with my index IS harder than playing a normal power chord in standard tuning. But like I said, it's not hard, just harder. Both are still quite easy.
    – user428517
    Jun 27, 2018 at 17:28
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A "power chord" is a combination of a root, fifth, and octave. These three pitches have a 2:3:4 frequency ratio. If the sounds are combined by any process which induces harmonic distortion, that process will add additional frequencies. If a signal contains two frequencies, harmonic distortion can add additional content at the sum or difference of those frequencies. In addition, the frequencies produced by harmonic distortion are then eligible to be added or subtracted from other frequencies to produce still more.

If a signal contains content which is concentrated almost entirely at frequencies in a 2:3:4 ratio or near-perfect multiples of those frequencies, then passing it through a distortion process will create many new frequencies, but all of them will be near-perfect multiples of half the original frequency. The net effect will be to produce a sound an octave lower than the original. While this technique is particularly popular in electric guitars, it has also been used in pipe organs for a long long time before that. The distortion effects organ pipes produce on the sound from nearby organ pipes tend to be much more subtle than those produced in electronic amplifiers, but the principle is the same.

To visually see why a power chord has this kind of effect, visualize the effect of adding together three signals with a 2:3:4 frequency ratio and clipping the result.

------______------______------______------______ root
----____----____----____----____----____----____ fifth
---___---___---___---___---___---___---___---___ octave
----____-___---_----____----____-___---_----____ combined and clipped

The top three waveforms represent the original signals (simplified to square waves for this illustration). The bottom waveform represents the sum of the first three. If the majority are high, the output will be as well. Likewise if the majority are low. Note that the output waveform has a complicated pattern, but it repeats at a frequency which is half of the original root frequency.

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    ASCII art for the win!
    – Eric Lippert
    Jun 27, 2018 at 22:12
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    On a pipe organ, this technique isn't about distortion. It's about approximating a very low note (say, C) which would need a big expensive pipe, with two smaller cheaper pipes (the C an octave higher, and the G above that). More of a "power note" than a "power chord."
    – Camille Goudeseune
    Jun 27, 2018 at 22:53
  • For those who'd like to see images of matching-up waveforms in somewhat better resolution than ASCII, I have some in this answer.
    – leftaroundabout
    Jun 27, 2018 at 23:21
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    @CamilleGoudeseune: The goal of a power chord is to generally to sound more like a rich low note than a chord. On a pipe organ, smaller mixtures use a group of pipes for each key (but the pipes in pedal mixtures are hardly small). The level of coupling in pipe organs doesn't cause much distortion, but any distortion that happens to be produced can enhance the effectiveness.
    – supercat
    Jun 28, 2018 at 3:39
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    You can play any neighboring tones from the harmonic series and it will behave like a power chord: After distortion, it contains all the other harmonics. Example: soundcloud.com/endolith/…
    – endolith
    Jul 10, 2018 at 1:35

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