Ambient / Diffuse / Specular

The basic lighting effect is diffuse lighting.

The intensity of diffuse lighting depends on the orientation of the surface relative to the light source.
Diffuse light is reflected equally in all directions.

Specularity is light that is more-or-less directly reflected off the surface to the camera.
Its intensity depends on the orientation of the surface relative to camera, as well as to the light source.

Ambient light comes from no particular direction.
It reflects equally in all directions