There are two basic forms of retaining walls; cantilever and gravity. Many cantilever retaining wall structure systems utilize cast-in-place steel-reinforced concrete. Using this type of retaining wall is able to "retain" the earth that is behind it by using its internal strength and rigidity.
Gravity retaining walls, on the other hand, are constructed of soil stabilized with man-made materials. This particular type of retaining wall has a degree in flexibility that allows them to adjust to minor amounts of differential settlement without suffering a large amount of structural damage.