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We will first discuss the structural divisions of the nervous system: the central nervous system and the peripheral nervous system. These divisions are often abbreviated CNS and PNS, respectively. The central nervous system consists of the brain and the spinal cord, while the peripheral nervous system consists of the nerves outside the central nervous system. A couple of useful terms to define at this point are nucleus and ganglion. In the central nervous system, a collection of neuron cell bodies is called a nucleus. In the peripheral nervous system, a collection of neuron cell bodies is called a ganglion (plural: ganglia). The one exception to this rule that you may have encountered is the basal ganglia in the brain. Indeed, the basal ganglia technically should be called the basal nuclei, but they were named prior to the terms being defined and the name has stuck. Remembering the difference between a nucleus and a ganglion, and that both are simply collections of nerve cell bodies, will help you as we add more information onto the nervous system. In the image, the red structures depict the central nervous system, and the blue structures depict the peripheral nervous system. While these branches are called "nervous systems," they are not actually separate systems. The central and peripheral nervous systems differ structurally and are therefore considered separately, but they work together and are both part of the same overall system.
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