Color: This channel applies color, textured patterns, or images to the surface.

Specular: This channel creates the highlights that appear on surfaces from reflected light. Some highlights are small and bright, like shiny metal, others are larger and duller like plastic.

Bump: This channel provides surface texture as most objects are not perfectly smooth. Bump requires a texture map or procedural shader of some sort. Darker areas can make an extrusion or depression on the surface. Rust ShaderTexture Map

Reflection: This will cause your object to show reflections of other objects on its surface. Reflection channel set on the floor object

Alpha: An alpha channel works similar to a mask in photoshop; black will block the image (making it invisible) and white will reveal the image. Using black and white images on an alpha channel allows you to create holes in objects.

By combing channels realistic objects and scenes can be created.

Demonstration project creating sample materials.