A digital elevation model' (DEM) is a representation of the topography of the Earth in digital format, that is, by coordinates and numerical descriptions of altitude. DEMs are used often in geographic information systems.
A free DEM of the whole world called GTOPO30 (30 arcsecond resolution, approx. 1km) is available (see Link below). It can be used to generate maps such as in the International gliding online contest. Several other DEMs are available, but the better they are, the more they cost.