Minecraft wiki entry for Walls_and_buttresses, associated text: A gravity wall is simply a thick wall that uses its great mass to hold back the earth.
A piling wall has reinforcing piles dug deep into the ground to give it strength. The piles may go down twice as far as the wall is high.
A cantilevered wall is shaped like an L. Some cantilevered walls are anchored by the weight of the soil burying the horizontal part of the L. Otherwise they may be supported by their own strength, or by stays or counterforts.
An anchored wall has cables or stays buried in the ground to help hold it in position.