Shading gives dimension to an illustration. It allows the viewer to observe a sense of volume and allows the artist to take the viewer more realistic understanding with object appearing three-dimensional. It also allows the artist to reveal the light source and can even give the viewer an idea of the setting of the piece. Many different techniques can be used to accomplish shading such as hatching, crosshatching, pointillism, blending, veiling, stumping, circular technique, and even scribble technique. In some cases an artist may even choose to work backwards by choosing a dark stock to represent the darkest point, and add a light colored medium to build to the highlight.
Samples:
Hatching
http://www.roymech.co.uk/images/hatch_ex.gif
Crosshatching:
http://th06.deviantart.net/fs36/300W/i/2008/243/e/b/Cross_Hatching_Sketch__Fist_by_esninism.jpg
Pointillism
http://www.robert-redmond.com/photos/NudeStudies/Pointillism.jpg
Blending
http://www.lifelikedrawing.com/5valuegrid.jpg
Veiling
http://www.explore-drawing-and-painting.com/images/shading-techniques-veiling.jpg
Stumping
http://www.explore-drawing-and-painting.com/images/shading-techniques-stumping.jpg
Circular
http://library.thinkquest.org/C005470F/images/fundamentals/shading1.jpg
Scribble
http://library.thinkquest.org/C005470F/images/fundamentals/shading2.jpg
Resources:
http://library.thinkquest.org/C005470F/technique/shading.html
http://www.explore-drawing-and-painting.com/shading-techniques.html