I disagree Phantast, at a simple level they are the same, both distort the base mesh along the surface normals according to the colour of the map used at that point. Displacement maps generally are used on bigger surface variations then bump maps. In high end applications either a temporary revised mesh will be created or a permananent revised mesh created, in the first scenario, the effect is that of a big bump map. The closest you'll get in bryce is a bump map, in the example veritas has given, using the map he's used as a displacement map as a bump map would get the same effect of scale edges etc. Dislacement versus bump is really a matter of how big the bumps are, if they're big effectively you need to change geometry, if they're small (as in this sample) then it's effectively a change in surface lighting unless very close to the surface being modified.

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.