DRG stands for Digital Raster Graphic, and DRG maps, along with DEM and DOQQ maps as well, are a type of topoquad, or topographic map, of the United States Geologic Survey(USGS). Technically, a DRG map is a scanned and georectified USGS topoquad. That means that since it's a topoquad, it's a map that displays elevation, topography, and other surface features. The same applies for DEM and DOQQ maps. Being that it is georectified, means that its map features have acquired latitude and longitude coordinates or UTM coordinates, tied to the real world coordinate system. The geographical information displayed by this map type is then conveyed in raster format, meaning the information is coded and presented in pixels. The above image is a DRG, because it is a raster image(pixels) of a scanned USGS topographic map, but is also referenced to a coordinate or UTM grid, which is made clear by the visible grid created on the map.
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