The root of the diminished 7th chord could be any of the notes. The chord is symmetrical, which means that if you where to move this chord up 3 frets, it would still be the same 4 notes, just in a different order. The diminished 7th chord has a lot of different uses. Of course the most novel use is to play the chord and then move up 3 frets and play the chord again. Keep doing this until you run out of frets. Did you just have a cartoon flash back? Another common use is as a passing chord between two chords which area are a whole step apart.

The o means diminished.