You may be referring to the difference between regular/traditional-style BCs and rear-inflate BCs.
The advantage to rear-inflate BCs is the air cell is behind you, where it should be, while you're diving and in a good horizontally trim position. It also won't squeeze you to the point where you have a hard time breathing, like some jacket-style BCs can.
And as it happens, Larry and Joe wrote up a little piece on this very question. See:
http://www.scubatoys.com/store/joelarry/backfloatation.asp