A power
chord is a basic, three
note chord consisting of a base note, a note one fifth above the base, and a note one
octave above the base note. Power chords are played mainly on the
guitar, and they are more often than not
distorted.
A power chord can also be thought of as the lower three notes of a 5 or 6 note barre chord. They are often the first thing a beginning electric guitar player learns to play. They allow for great, thick sounding melodies to be played on the guitar, but they do not contain much texture or dissonance.
Although you can find them all over modern music, a famous example is Nirvana's Smells like Teen Spirit. The introduction consists of 4 power chords, repeated. First it is clean, then later distorted.
Some variety can be used with power chords, although it is not done often. In Tool's Sober, the main rhythm is a power chord hit twice in a row quickly, a rest, then the same chord/rhythm repeated, except the middle note is a minor sixth instead of a fifth.