LEDs differ from your traditional light sources in the way they produce light.

In an normal incandescent lamp, a tungsten filament is heated by an electric current until it glows and emits light.
In a fluorescent lamp, an electric arc excites mercury atoms, which emit ultraviolet (UV) radiation. After hitting the phosphor coating on the inside of glass tubes, the UV radiation is converted and emitted as visible light.

An LED, in contrast, is a semiconductor diode, an electronic component.
It consists of a chip of semiconducting material treated to create a structure called a p-n (positive-negative) junction.
When connected to a power source, current will flow from the p-side (anode) to the n-side (cathode), but not in the reverse direction.
Charge-carriers (electrons and electron holes) flow into the junction from electrodes. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon (light).
The specific wavelength or color emitted by the LED depends on the materials used to make the diode. Red LEDs are based on aluminum gallium arsenide (AlGaAs). Blue LEDs are made from indium gallium nitride (InGaN) and green from aluminum gallium phosphide (AlGaP). "White" light is created by combining the light from red, green, and blue (RGB) LEDs or by coating a blue LED with yellow phosphor.