Basically it is a perforated plate or series of joined perforated tubes which is placed under the aquarium gravel. Water is drawn through the gravel, through the plate or tubes, up through a lift tube, and finally recycled back on top of the aquarium water.
This type of filter involves a few long plastic tubes, which remain upright in the aquarium and are attached to a plate on the bottom. In each tube is an air stone attached to an air pump. As the air bubbles rise, a current is generated which continuously brings water from the aquarium, through the substrate and to the top of the tubes.