An LED is a small, yet bright, light made of a tiny crystal that emits a color when electricity is connected to it.
A typical LED is shown in  Figure 2.4. The left-hand side of the diagram shows a physical representation of a LED, while the right-hand side shows the schematic symbol for a LED:
LEDs need to be connected the correct way around into a circuit, otherwise, they will not work. If you look closely at your LED, you will notice a flat side on the LED casing. The leg on this side is the cathode, which connects to the negative or ground side of a power source. The cathode leg will also be the shorter of the LED's legs. The other leg is known as the anode and connects to the positive side of a power source. If you examine the LED symbol, you will notice that the cathode side of the LED has a line drawn across the tip of the triangle...
