From ZBrush Site:

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Displacement Maps

ZBrush can compare your original mesh to the high frequency model in order to create a grayscale difference map. To do this, you simply import your original mesh back into the lowest subdivision level and then click the Create DispMap button. Alternatively, if the shape of the original low-res mesh needn't be preserved, press the Cage button to generate a base-level object ideal for the calculated map. This cage can then be exported along with the map.

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Normal Maps

These maps use colors to tell a rendering engine how to perturb the surface normal of an object. Although this does not change the object's silhouette, it does allow remarkably realistic depth effects that are superior to bump mapping.

ZBrush can use Tangent space or Normal space (local and global coordinates), and also has modifiers in the Preferences palette to match how your rendering engine uses the colors in the map