The nervous system is a very important part of the body because if we didn't have it, we would not be able to do anything.
The nervous system starts with the brain, which is connected to the spinal cord, which goes down your spine. This is why a spinal injury is so serious. All the nerves are attached to the spinal cord which is attached to the brain.
The spinal cord and brain are called the central nervous system and the nerves and other organs are called the peripheral nervous system.
The nervous system has two functions: to transmit signals from the brain to your body and to transmit signals from the nerves to your brain. When the brain is controlling the body, you need to actually think about what you're doing. Most of the signals are automatic like the brain signaling your lungs to breathe or your heart to beat.
When you do actions (like running), your body sends signals to your brain that it needs more oxygen, so the brain "tells" the lungs to breathe faster for more oxygen.