The nervous system is the collection of cells and tissues that form the structures and organs involved in collecting and processing sensory information and then triggering reactions.
The nervous system is broken down into the central nervous system and the peripheral nervous system. The central nervous system is just the brain and the spinal cord. The peripheral nervous system is the rest of the nervous tissue in the body and the sensory organs that the nervous tissue attaches to. The peripheral nervous tissue includes the cranial nerves that branch out from the brain and the spinal nerves that branch out from the spinal cord, as well as all the sensory organs in the head and body.
Surprisingly, most diagrams of the nervous system leave out the sensory organs of the body and include just the brain, spinal cord, cranial nerves, and spinal nerves. Figure 10-1 is a typical diagram of the nervous system, showing the central nervous system in yellow, the nervous tissue of the peripheral nervous system in blue, but leaving out all of the sensory organs of the peripheral nervous system. Of course, some anatomists categorize the sensory organs as their own organ systems, yielding the ocular system (eyes), gustatory system (mouth and nose), auditory system (ears), proprioceptive system (touch), etc. If you consider the sensory organs parts of their own systems, it makes sense to leave them off diagrams of the nervous system.