Definition: Light emitting diodes are diodes (electronic components that let electricity pass in only one direction) that emit visible light when electricity is applied, much like a light bulb. When many LEDs are side-by-side, they can create pictures, such as the scrolling red LED signs found everywhere.
What's inside them?
LEDs are simply diodes that are designed to give off light. When a diode is forward-biased so that electrons and holes are zipping back and forth across the junction, they're constantly combining and wiping one another out. Sooner or later, after an electron moves from the n-type into the p-type silicon, it will combine with a hole and disappear. That makes an atom complete and more stable and it gives off a little burst of energy (a kind of "sigh of relief") in the form of a tiny "packet" or photon of light.
LEDs are specifically designed so they make light of a certain wavelength and they're built into rounded plastic bulbs to make this light more concentrated and brighter. Red LEDs produce light with a wavelength of about 630-660 nanometres