It's strange how many people get lost in the difference between power chords and bar chords.
For those people I will start off by saying power chords have 3 fingers on 3 strings and bar chords have 1 finger on ALL strings and chords using you pinky, ring and middle finger.
Let's go onto a power chord called F5.
That is a power chord, I will now show you a bar chord called
F...
Power chords are known to be used in rock music like;
Nirvana,
Blur,
Foo Fighters,
Pearl Jam etc...
You can still use bar chords in rock music just power chords give a 3 note sound.
Hoped I helped,
Scotty
Either you don't know what your talking about or you suck at definitions. You don't have to use three fingers, Drop D allows you to play some Power Chords using one finger.
Not only this but you don't HAVE to use three fingers across three strings, two string power chords are perfectly acceptable.
Again, with Bar Chords you don't have have to press ALL the strings, only for certain chords.