From The Art and Popular Culture Encyclopedia
The nervous system is a highly specialized network whose principal components are nerves called neurons. Neurons are interconnected to each other in complex arrangements and have the property of conducting, using electrochemical signals, and a great variety of stimuli both within the nervous tissue as well as from and towards most of the other tissues. Thus, neurons coordinate multiple functions in organisms.
Nervous systems are found in many multicellular animals but differ greatly in complexity between species.