Strictly speaking, fifth chords (aka power chords) and tritones (diminished fifth chords) 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 fifth chords lack the third in comparison to major or minor chords and have a very stable sound, they are perfectly suited for riffs with distortion, which has earned them the name power chord. Tritones, however, are the pure opposite with their diminished fifth. They have a very unstable sound that strives for resolution and was feared in ancient music as diabolus in musica.