A nerve is a cell that is specialized for sending and receiving information. Nerves make up the part of your body that tells your brain what your body is doing, called the peripheral nervous system. Your muscles, your stomach, and even your heart would not function if a nerve didn't send it directions from your brain.
Nerves carry signals like wires carry electricity. The long nerves in your body are like wires and the current would be the signals carried in the nerved. Charged atoms called ions carry nerve signals. The ions move in an out of the nerve cells in a wave-like manner down the nerve. This causes a charge to move down the nerve, this is how a nerve signal is carried. The longest nerve is the body is called the sciatic (si-at-tik) nerve and it is in your leg.
The sciatic nerve is a single cell that begins in your lower back by your spine and runs to the heel of your foot! Some nerves only send instructions from your brain to body parts; other nerves are there to report back to the brain on what is happening to your body. A nerve can actually sense changes in temperature, pressure, pain, or light, if the nerve has the right molecules. That's one exciting area of neuroscience (the study of brain stuff): neuroscientists are trying to figure out what types of molecules are found in each different nerve in your body. If you know what molecules are there, then you have a pretty good idea of what each nerve does.
When nerves are in your brain or spinal cord, we call them neurons instead. These neurons are part of the central nervous system. Neurons are a bit more complicated because instead of just sending information from one place to another, like a nerve, each neuron makes thousands of connections to other neurons. This means that the information can be sent backwards, forwards, sideways, even back in circles inside your head. We think that thinking has a lot to do with the complex pattern of information flowing in your brain. And when you think that these patterns in your brain are a million times more complex than the circuitry of the fastest supercomputer, your brain might just fry trying to comprehend its own complexity.
Consider this. Look up at the sky tonight. Try to count all the stars you can see without counting the same one twice. Now, imagine that each of these stars has nine planets like our solar system, and that each planet has nine moons orbiting it. Now imagine if you could draw lines connecting every moon on every planet to every other moon, creating some sort of a web across the sky. This "web" would look a little like the connections between all the neurons inside your brain.
However, your brain is even more complex than that! Your brain actually contains 10,000,000,000 connections between all the neurons, which is a network so complex, you would have to connect all the stars in the galaxy, including all those you can't see when you look up at the sky, to paint a picture like the complex web inside your head.