Basically the peripheral nervous system (PNS) consists of all nerves and ganglia which are not in the central nervous system (CNS) which consists of the brain and the spinal cord. The PNS can be divided in to the autonomic nervous system (ANS) which controls involuntary functions such as heart rate and the production of saliva and the somatic nervous system.
ANS can be further divided in to two categories called the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). SNS and PNS are integral in the homeostatic regulation of the bodies functions.
The somatic nervous system is associated with the bodies voluntary movements such as those of the skeletal muscles, ie the act of you picking up a pen. It is also associated with receiving external stimuli ie hearing, sight and touch.