In 3D shaders are extremely important in giving polygons more information and to flesh out an object, rather than it being just a simple flat image.
When a polygon starts out, it has no values or anything overly distinguishable other than often a single colour. Shaders add the details, colouring and material to the polygons and without them they will be mostly lifeless.
The 3 main shaders used are:
Colourmaps are exactly as they are implied. They add colouring to the polygon surface, whether it is a hand painted texture or a photograph. The colourmap sets the overall colour of the object.
Specular Maps are another layer added to the colourmaps to bring out reflective highlights. The highlights are depicted by a black and white image, where the whitest values are the lightest/most reflective and the blackest values are the dullest/ least reflective.
Bump/Normal maps are a shader used to give depth to the colourmap and show details in the surface. A bump map works similar to speculars, in which the whites are higher surface peaks and black is a deeper surface. Normals are a more accurate version of bump maps and allow for greater detail, but rather than using black and white it uses Red, green and blue allowing more angles to be picked up. Generally normal maps are extracted from higher polygon meshes and baked onto a lower poly version giving the illusion of the lower poly model having more detail than it does.
If we were to make a golf ball, we would use a basic white texture for the colourmap, a patterned bump map of small circles repeated and then lightly highlighting the ball with a specular with a light amount of noise/dither to bring out the shine ( or lack of shine ) on the ball. To add even more detail, a secondary colourmap can be added with dirt and marks. The dirt and marks can then be softened/randomised by creating an alpha channel and painting white/black for where the ball should be dirtier/scuffed.