Articles like the wikipedia entry for the Forearm distinguish between superficial, intermediate, and deep muscles. What is the difference between these muscle levels? What are some readily recognizable examples of each? Are deep muscles more significant or powerful than superficial muscles?
It simply has to do with the location of the muscle in relation to the skin surface vs. bone.
See Superficial Muscles of the Human Body for more examples.
Superficial muscles are the most visible, so body builders will spend time to make sure they look right. However, the deep muscles and intermediate muscles add to the overall mass of a person, as well as improving their ability to lift heavy things. All strength athletes (from body builders to Olympic weightlifters and power lifters) spend time developing the muscles you cannot see. Compound lifts are an important component of this process.