Displacement is similar to bump mapping, with the exception that the geometry mesh is actually moved (and depending upon the method, additional geometry is created) to the target defined by the displacement maps and settings. Bump mapping fails in parallax, as well as in the silhouette (outline) of the rendered object. Here you can see that bump mapping is a trick. Displacement mapping, however, is the real deal. Its always a good idea to try bump mapping first, and then use displacement mapping as a fall back, since displacements can strain the resources you have. In particular, sub-pixel displacement (SPD) can be a big memory and time hit when rendering.