It is already understood that nervous system controls various bodily functions. The simplified form of functions controlled by nervous system are the followings:

1. Contractions of voluntary muscles to move the joints or to move some organs like tongue, eyeball.
2. Contractions of involuntary muscles like:
   
   1. Visceral muscles.
   2. Smooth muscles in the wall of cardiovascular channel.
   3. Smooth muscles in the root of hair follicle of skin, known as Arrectores pili.

3. Secretions of exocrine glands which may be either single, large, solitary, e.g. Salivary glands or tiny innumerable, for example–mucous glands of gastrointestinal and respiratory tract.

Out of these different functions—The contractions of voluntary muscles is controlled or regulated as per one’s own desire and is known as voluntary function, whereas others are not under one’s own control, called involuntary function.With the help of this background knowledge, it is to be noted that — functionally the nervous system is classified as — Somatic and Autonomic.

1. Somatic Nervous System: It is that division of nervous system which controls or regulates the voluntary functions, i.e. functions which can be performed as well as controlled as per one’s own desire. It is contraction of voluntary or skeletal muscles.
2. Autonomic Nervous System: It is that division of nervous system which controls or regulates involuntary functions, e.g. functions which can neither be preformed nor can be regulated as per one’s own desire. These are contraction of involuntary or smooth muscles and secretion of exocrine glands.

Two parallel components of autonomic nervous system:
They are called sympathetic and parasympathetic nervous system. These two systems have anta-gonistic actions on the “same” target organ, e.g. Parasympathetic nervous system contracts the muscles in wall of hollow viscera like GI tract (peristaltic movements), but relaxes the sphincters; whereas the sympathetic nervous system causes the opposite action on the same target organ. Again in some cases either of them has the influence, e.g. mucous glands of respiratory or alimentary tract are under control of parasympathetic nervous system, whereas secretion of sweat glands are controlled by sympathetic system.

Source: Easy and Interesting Approach to Human Neuroanatomy (Clinically Oriented) (2014)