Displace
The displace modifier lets you use a black and white image or map to displace the mesh. Strength determines the magnitude of the effect, if you set a very high magnitude you may need to increase your objects base subdivisions to maintain a smooth appearance.
The above plane was displaced using the modifier and a checker map. The plane has 128x128 subdivisions to give it a smooth appearance. If you scroll down further on the displace modifier rollout you will see more options for the map, such as U V W tiling etc.
The above checker map could be used to create a tiled wall, but there are way to many polygons just for simple, square extrusions. We can fix this by using the optimize modifier.
On the left the optimize modifier has not been applied, where as on the right it has. This significantly reduces poly count whilst maintaining the desired effect to a high degree.
Displace could be used for many effects, the most popular is terrain. It can also be used to create damage effects.