Lighting - Introduction to 3D Game Programming with DirectX 9.0 - page 110

Figure 5.8: A screen shot
from the DirectionalLight
sample
5.7 Summary
Direct3D supports three light source models: directional lights,
point lights, and spot lights. Light sources emit three types of light:
ambient light, diffuse light, and specular light.
The material of a surface defines how light interacts with the sur-
face that it strikes (that is, how much light is reflected and
absorbed, thus determining the color of the surface).
Vertex normals are used to define the orientation of a vertex. They
are used so that Direct3D can determine the angle at which a ray of
light strikes the vertex. In some cases, the vertex normal is equal
to the normal of the triangle that it forms, but this is not usually the
case when approximating smooth surfaces (e.g., spheres,
cylinders).