How do I use this, and what is it usually used for?
You use it by enabling it in the appropriate texture dimensions.
i.e. glEnable(GL_TEXTURE_GEN_?);
where ? is S, T or R.
Enable S and T for sphere mapping. Enable R as well for cube mapping.
The method that S, T, and R will use to determine how the texture coords are generated is specified by glTexGeni(GL_?, GL_TEXTURE_GEN_MODE, GL_X);
Where GL_? is GL_S, GL_T, and GL_R, and GL_X, is one of GL_SPHERE_MAP, GL_EYE_LINEAR, GL_OBJECT_LINEAR, or GL_REFLECTION_MAP (cube mapping)
check out my website… for some example code of using this.
www.nutty.org
Nutty 