Displacement map actually deforms the mesh along its deformation references usually a grayscale bitmap and can be deformed to a very high degree even to the point of complete physical (in a virtual sense) distortion of the mesh. Best effects are achieved with high poly count meshes for better defined visual effects. Whereas bump maps do not deform the mesh at all but promotes the illusion of bumps using height map information namely a grayscale bitmap in conjunction with the direction of the light. What usually determines which one to use depends on the closeness of the viewers POV. Since deformation usually require high poly counts this means it should be used sparingly to avoid long render times. Oftentimes objects in the distance would fair just as well with a bump map without the added cost of longer render times. But when youre in your face close to an object, no doubt the actual 3D deformation pays off in visual devidents. My 2 cents.