Normal mapping is another approach  for adding more detail to a polygonal object without actually adding polygons; mostly it is game technology.  Displacement mapping seems to be much more popular and widely-used.  A big difference is that normal mapping can give the bump effect in x, y, and z directions (using all 3 color channels) while displacement mapping only effects one axis (+/- z) from the particular normal of the polygon the map is laid over.  Another big difference is normal mapping does not actually bring the surface of the normal up or down at render time, while displacement does; so shadows will tend to be more accurate with displacement mapping, and other caveats may apply.

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