 DEM is often used as a generic term for DSMs and DTMs, only representing height information without any further definition about the surface.Other definitions equalise the terms DEM and DTM, equalise the terms DEM and DSM, 
define the DEM as a subset of the DTM, which also represents other morphological elements, or define a DEM as a rectangular grid and a DTM as a three-dimensional model (TIN).Most of the data providers (USGS, ERSDAC, CGIAR, Spot Image) use the term DEM as a generic term for DSMs and DTMs.