An LED is a light-emitting diode. Lighting an LED is considered the "Hello world" of a circuit design, and it can be as simple as putting a series resistor or can get more complicated, involving PWM and multiplexing.
It's the most common optoelectronic device, which generates light from the electron-hole recombination mechanism.
LEDs have long been used as indicator lights, but in recent years high power LEDs are being used as incandescent lamp replacements.
Theory of operation
The color is determined by the energy gap between conduction and valence band of the semiconductor used, which determines the energy that the photon generated by the recombination mechanism has.
Being a diode, it's characterized by a threshold voltage (of about 1.8 to 5 V depending on the color), below which the current is nearly zero, and beyond which the current increases in an exponential fashion. In a first approximation, the voltage is considered almost constant and equal to the threshold.
Most LEDs require a current limiting, for which usually a series resistor is used.
- For questions on a specific part, please link to the datasheet.