Guitar chord playing technique consists of movable patterns. Power chords are easy because the player only has to play two notes. The root note is played on the low E string with the index finger, and the fifth is played with the middle finger on the second lowest string, the A string, two frets up from the index finger. Or the ring finger can play the fifth and the octave, which is next to the fifth on the third lowest string, if the player can depress two strings without creating a buzz. Power chords don't have much variety, but there are many different patterns for bar chords, using all or few of the fingers to create a large assortment of voicings that make the chords more interesting.
The definition of a chord is that it is a series of three or more notes. Power chords are actually not chords at all, because they consist of only two notes - the root and its fifth. If you play a C power chord you'll play the C and the fifth note in the C scale, which is G. If you play an A power chord, its fifth is an E.
Bar chords offer much more depth to music. All six strings can be played to create complex chords, such as a major, which consists of the root, third and fifth; the minor, which is the root, flatted third and fifth; the major and minor seven, which adds the seventh note or the flatted seventh; the suspended fourth, which adds the fourth, and the ninth, which adds the second note after the higher octave.
New guitar players may not have developed the hand strength to lay their index finger across all six strings and play clear notes. They also may not have the dexterity to move the other fingers onto other strings while keeping the bar intact. Power chords offer the satisfaction of creating a full sound in the music with little education and hand strength.
Power chords are found most prevalently in metal music, but are not confined to it. In order to make the power chords sound full the player uses effects, such as distortion, which is characteristic of metal music. Since power chords play the root on the lower E string and don't make use of the upper strings the chords are low in sound.
Bar chords can be used in any type of music, as is evident in the beautifully intricate guitar playing of the metal band, Metallica, rock guitarist Carlos Santana and flamenco southwest style of Ottmar Liebert.