A DEM (digital elevation model) of Difference (DoD), is the result of DEM differencing. DEM differencing is the simple act of subtracting the elevations of an older DEM from the elevations a newer DEM on a cell by cell basis:

This video lecture covers the basics of DEM Differencing:

• For more information, see pages 59-61 of in Chapter 3 of:
  
•  Wheaton JM. 2008. Uncertainty in Morphological Sediment Budgeting of Rivers. Unpublished PhD Thesis, University of Southampton, Southampton, 412 pp.

References:

• Bangen SG and Wheaton JM. 2012. CHaMP Crew Variability: Influence on Topographic Surfaces & Derived Metrics, Report to Eco Logical Research, Inc. and the Columbia Habitat Monitoring Program, Logan, Utah, 79 pp.