LEDs are solid-state semiconductor devices that convert electrical energy directly into light. Thermal sources of light such as flames and incandescent filaments emit light when heated, either by chemical reaction (flames) or electrical heating(filament lamps).
LED "cold" generation of light leads to high efficacy because most of the energy radiates within the visible spectrum. Other common high efficiency light sources, such as fluorescent lamps and electro luminescent devices, also produce light without much thermal radiation outside the visible spectrum.
Because LEDs are the solid-state devices, they can be extremely small and durable; they also provide longer lamp life than other sources.