A pulk is an adapted sled that you pull behind you to haul your gear. It allows you to haul more gear with less effort, taking the weight off your body and legs. During the winter it becomes a huge asset.
The document below is available here and does a great job of summarizing build options and features. A quality pulk can be built for far less than a ready-made solution. Here are two sleds which I would recommend as a base for a pulk build: Emsco Beast and Paris Expedition. Have fun!