The downpipe is the piece that goes from the turbo to the middle of the car. In the OEM version it comprises the ceramic pre-cat (roughly 2 centimeters after the turbo), the flex, the metallic cat (roughly 50 centimeters after the pre-cat) and a double piping that is meant to reduce (not eliminate) the popping sounds. The Milltek downpipe for road use comprises only the flex and the metallic cat (200 cpsi).
The cat-back part of the exhaust is composed by the center silencer and the final silencer, in the OEM version you halso have the exhaust valve that is eliminated with the Milltek version.
The difference between resonate and non-resonated (the one I have) is that the non resonated, instead of the center silencer, has a straight pipe. Hence the non-resonated version is louder that the resonated one.