A screw conveyor is a piece of equipment used in various industrial applications to transport material. Generally, the material falls into a trough with a large, horizontal screw. As the screw turns, the grooves in it push the material along the trough. A screw conveyor is often used to evacuate the dust collected in a precipitator.
A good visualization of how the mechanism works would be a snack food vending machine. The potato chips, Twinkies, crackers, cookies, etc. are placed along a coil. As the coil turns, the foodstuffs are pushed along to eventually fall down the front of the machine into the retrieval bin.
Screw conveyors are a useful way to move material when the surrounding environment is harsh (a metal screw can withstand more than a belt conveyor) and over relatively short distances.