An LED, or light-emitting diode lightbulb, is a bulb that produces light when electricity is passed through a semiconductor. Because of this, LEDs are more efficient than fluorescent lights and far more efficient than incandescent lights, which squander most of their energy as heat.
LEDs are also cooler than traditional lights because the heat the bulb produces is absorbed then dispersed by a heat sink. They also differ from incandescent and fluorescent lights in that they are directional. This makes them excellent for task lighting but not ideal for general lighting.
LED lightbulbs don't burn out the way incandescent lightbulbs do. The light they produce simply decreases over time.