Oh, and is a warp field and subspace field the same thing?
Depending on what theory you subscribe to, a warp field is made up of many layers of subspace fields, with propulsion achieved by pushing off them. The more subspace layers being pushed off of, the faster the ship goes at warp flight, IMO.
Another theory is similar to the one above, but involves the ship going continually deeper into the subspace domain. The deeper the ship goes into subspace, the higher the warp factor, etc.
In both, the ship's actual speed remains slower-than-light within its warp envelope.