Shading is made up of shadows and light. If you think about where the light hits the object, that will affect your shading.
Useful exercise: draw a circle. Pick a direction the light is coming from. Then start shading the circle to look more like a rounded object. The spot closest to the light will be the lightest. The further away from the light-the darker it is. [IMG]file:///C:/DOCUME%7E1/MARKAL%7E1/LOCALS%7E1/Temp/moz-screenshot-13.jpg[/IMG]When you're finished shading, your plain circle will look like a 3-d object...all because of how you used light and shading. http://www.atpm.com/9.07/images/design-spheres.gif
In the case of a horse-Ilovesaddlebred's pictures were great. And it's the same concept. Where the light hits will be the lightest. And you can tell the muscle definition in the neck because of the shading. So not only does shading show the light and dark aspect of a drawing, it also shows the anatomy of the horse. (It helps to study the anatomy of horses to know their bone structures and muscle definition...something I'm still struggling with
I know it sounds complicated. But if you start with basic shapes, ie circles and rectangles, you'll get the basic feel for shading. Then there's nowhere from there but forward