As Ian said, an open tuning is one where the open strings form a chord.
My favorite tuning is D-G-D-G-B-D. It's Open G. Why use it? It's better than Open D for almost everything. You can slide into the root from below (unlike Open D). You get a minor third at the top. And you can still do one-finger power-chords, but you have to do them on the "A" string (standard).
It also gives you "more" Octaves. If you play a little melody across a few strings, it is much easier to find the octave transposition. The Open chord is very "5"-heavy giving it an unstable, edgy effect.
For a while I dropped the whole thing down a whole step producing C-F-C-F-A-C. But then, everything's too low to sing properly. But I often still write with a C-clef which was quite natural for this tuning.
