A D5 (D Power Chord) chord contains the notes D, A. It is built from the formula 1 - 5, with D as the root.
Why do so many rock songs sit on a single power chord?
Because a power chord has no third, it never commits to major or minor, so your ear doesn't tire of it the way it would of a full chord. That harmonic neutrality is what lets a riff repeat for bars on end and still feel like it's driving forward.