Short for light-emitting diode, LED is a semiconductor that illuminates when an electrical charge passes through it. LEDs are commonly green, amber or red; however can be an assortment of other colors. Below are examples of how an LED could be used with a computer.
A good example of an LED is the led status indicators on your keyboard for Num Lock, Caps Lock and Scroll Lock as shown in the picture. In this picture, the blue LED on the keyboard indicates the Num Lock is enabled, but since the Caps Lock and Scroll Lock are not on it indicates they are not enabled. The location of these indicators depend on manufacturer of the keyboard, but are usually located in the top center or top-right hand side of the keyboard.
Not all keyboard num lock, caps lock, or scroll lock indicators have identifiers like those shown in the picture. If you are not sure what LED goes to what key, press the num lock, caps lock, or scroll lock key while watching the LED's and see which one illuminates.
Apple keyboards and some other keyboards have the Caps Lock LED indicator on the Caps Lock key and may not have other indicators because there are no scroll lock or num lock keys.
Most computers today use an optical mouse, which is a mouse that uses an LED to help track the cursor on the screen.
Many motherboards also have an onboard LED that lets anyone looking at the motherboard know that there is power getting to the motherboard and in some cases if there are errors. Pictured at right is an LED/PLED located between the PCI slots on a motherboard.
Do not work inside a computer or disconnect any internal plugs while this LED is lit.
When the power cable is disconnected from the computer the LED light stays on for several seconds and then slowly fade off, which is normal.