It's easiest to understand the organization of the nervous system (NS) if we consider the things that the NS must do:

Some terminology first

Neuron
Nerve: group of axons in the PNS (white matter)
Tract: group of axons in the CNS (white matter)
Ganglion: group of cell bodies in the PNS (gray matter)
Nucleus: group of cell bodies in the CNS (gray matter)

Starting with the basic functions of the Nervous System:

• Controlling the Internal Organs (respiratory system, digestive system, reproductive system, etc.). Carried out by the Autonomic Nervous System
• Sensation: detecting simple stimuli from the environment recieved by the sense organs (eyes, ears, nose, tongue, skin). Cranial Nerves and Spinal Nerves
• Movement: contracting muscles in response to stimuli. Cranial Nerves and Spinal Nerves

We've just described the primary functions of the Peripheral Nervous System.

PNS = Somatic Division (Cranial Nerves and Spinal Nerves); Autonomic Division (Sympathetic NS and Parasympathetic NS). The PNS does not function autonomously, however...it communicates bi-directionally with the neurons of the CNS. The somatic division communicates with the perceptual and motor control regions of the brain; the autonomic division is under control of brain stem nuclei, including the hypothalamus.