A C5 (C Power Chord) chord contains the notes C, G. It is built from the formula 1 - 5, with C as the root.
When would I use an A-string power chord instead of a low-E one?
Mostly for two reasons: the note you want is simply closer on the A string, or the riff needs to cut rather than rumble. Sixth-string power chords carry more low-end weight; fifth-string ones sit above the mix, which matters when a bassist is already occupying the bottom.