Focus stacking is a way of increasing the DOF in a photograph by combining several pictures taken from the same point of view but at different focus points. It is normally applied in macrophotography where there is a constant battle between light, aperture, DOF and diffraction softening but can equally be applied in other photographic fields if lack of DOF is causing problems.
I 've done a tutorial here
Wonderful Photos: How to Increase your Depth of Field by Focus Stacking
on using the freeware PC programme combinezm.
Brian V.