Relief shading involves the use of shadows to emphasize the topography of a terrain. In GIS, most often this involves the use of a digital elevation model (DEM) that has undergone a technique called hillshading. The variation in shadowing gives the elevation a 3D effect and helps put into context how hilly or flat a geographic area is. There are different methods for achieving a shaded relief map and resources for understanding this cartographic technique are listed here.
Brief overview of hillshading along with some discussion on hand hillshading and methods. Although the page is dated, it still provides some useful base information.
Examples and methodology for creating 3D illusions of terrain using hachure, point symbol and inclined contour techniques.
Comprehensive site devoted to the techniques of shaded relief maps.
Web site of Tom Patterson, from the United States Park Service which presents examples and articles on shaded relief techniques.
Hillshading of terrain using layer tints with aspect-variant luminosity.(cartography): An article from: Cartography and Geographic Information Science [HTML] (Digital)
by Patrick J. Kennelly (Author), A. Jon Kimerling (Author)