Power chords are movable two note “chords” played on two or more strings on a guitar.

On an distorted electric guitar, power chords creates a strong sonic effect. An power chords are used because the allows for a much higher levels of distortion without causing dissonance.

Trick: power chords can be used to fill in between difficult chord changes.

Power chords are rudymentary chords, not built from minor or major third intervals like proper chords. They consist of a perfect 5th interval (or its inversion. Note: The inversion of a perfect 5th inteval is a perfect fourth). The root note can be doubled at a higher pitch, an octave, called octave doubling.

Possible interval patterns / voicings: 1-5, 5-1, 1-5-1, 5-1-5-1 

                           
   F5         F5         A5   
 EADGBE     EADGBE     EADGBE 
 R|||||     ||||||     |||||| 
 ||||||     ||||||     ||0R|| 
 |50|||     |5R|||     |||||| 
 ||||||     ||||||     |||||| 
 Fcf         cF        ||||50 
                         eAea 
 R = Root note

Notice where the root note is in the power chord patterns. Recognize that the inverted power chord consists of a perfect 4th interval, and sounds quite distinct and different.

As noted, power chords lack a third. To see this, compare the F5 chord to the F Minor and F Major chords on your guitar, an take notice of where the third is added. Also, notice that when you don't play the third, you are not saying if it's a minor or a major. You are not expressing a mood so to speak, or the mood is ambiguous: 

   F5       Fm        F    
 e|x       e|x      e|x    
 H|x       H|x      H|x    
 G|x       G|1      G|2    
 D|x       D|x      D|x  
 A|3       A|3      A|3    
 E|1       E|1      E|1    
            
  F5        Fm        F    
EADGBE    EADGBE    EADGBE 
R|||||    R||0||    R||||| 
||||||    ||||||    |||0|| 
|ea|||    |00|||    |00||| 
||||||    ||||||    |||||| 
 
 1-5      1-b3-5    1-3-5  
 F-C      F-Ab-C    F-A-C

Because it doesn't have a third, it can be used both over major and minor scales and chord changes: 

 Harmonized F Major Scale with Power Chords 
 
     I   II  III  IV   V   VI  VII  VIII 
     F5  G5  A5   Bb5  C5  D5  E5   F5 
 e-----------------------------0----1---| 
 B---------------------1---3---0----1---| 
 G-------0---2----3----0---2------------| 
 D---3---0---2----3--------0---2----3---| 
 A---3-------0----1----3----------------| 
 E---1---3------------------------------|

Notice that you can use this to learn to recognize the position of the 3rd in some scales. 

       The famous Smoke on the Water riff 
  
    G5-A#5-C5        C#5                                     
 e------------------------------------------- 
 B------------------------------------------- 
 G--0--3--5-------3--6\5------3--5---3------- 
 D--0--3--5----5--3--6\5---5--3--5---3-5----- 
 A-----1--3----5--1--4\3---5--1--3---1-5----- 
 E--3----------3-----------3-----------3----- 
  
      Simplified - inverted chords 
 e----------------------------------------- 
 B----------------------------------------- 
 G--0--3--5---0--3--6\5---0--3--5---3-0---- 
 D--0--3--5---0--3--6\5---0--3--5---3-0---- 
 A----------------------------------------- 
 E----------------------------------------- 
  
 This is the the **Smoke on the Water** riff that 
 appeared on //Deep Purple//'s album //Made in Japan//.