Diffuse Maps

A diffuse map of a door.

A diffuse map is a texture you use to define a surface's main colour.

In order to work well with a normal map and a specular map, a good diffuse texture should not have any directional lighting included, it should only have generic "ambient occlusion" - ie. the surface gets darker in deep cracks and around embossed details. If you are generating your normal maps from high-poly geometry, you should bake a matching ambient occlusion pass from the geometry and multiply this on top of your diffuse texture to make sure that the lighting matches the normal map.