A tapered steerer uses a larger bearing at the bottom than at the top, so as to improve rigidity and bearing life without needing a new larger standard stem.

Generally, once you go tapered fork and frame you stay with it, but there may be an adapter headset for fitting a 1-1/8" standard fork into a frame made for a tapered fork. You can't go the other way because the bottom of a tapered fork is too large for a standard head tube.