Strictly speaking, the fifth chord (aka power chord) and the tritone (diminished fifth chord) are not chords, but intervals that consist of only two notes. In the fifth chord, these are the root and the perfect fifth, while in the tritone, the fifth becomes diminished (flatted). By means of octavation, however, additional notes can be picked, so that the full sound of a chord is created.
Since the fifth chord lacks the third in comparison to the major or minor chord and is a very stable sound, it is perfectly suited for riffs with distortion, which has earned it the name power chord. The tritone, however, is the pure opposite with its diminished fifth. It has a very unstable sound that strives for resolution and was feared in ancient music as diabolus in musica.