A light-emitting diode (LED) is a light source that emits light when an electrical current is applied to it. Discovered in the early 20th century, the technology has been greatly developed and continues to advance through research and development. From early indicator lights with low light output--with only one available color--to today's devices that emit visible, ultraviolet or infra red light, with very high brightness.
The technology behind LED is based on semiconductor technology, which is also the basis of modern computers. In the semiconductor diode, electrons are brought from a state of high energy to a state of low energy state and this energy difference is emitted in the form of light, the effect is called electroluminescence. Specific colors are associated with specialized materials, that are constructed to have an energy gap corresponding to light with particular wavelength/color. An LED is usually a small area (less than 1 mm2) light source, often with optics added directly on top of the chip to shape its radiation pattern and assist in reflection. 
LEDs have many advantages to traditional light sources, such as: Low energy consumption, longer lifetime, robustness, small size among others. However they still remain relatively expensive, and have some characteristics that differentiate them from traditional light sources, such as need for current- and heat management. These advantages have caused LEDs to be used in many new applications where traditional light sources could not be used, as well as traditional applications where especially the low energy consumption is appreciated. Despite high price and the need for specialized design, LEDs are seeing adoption in more and more areas of lighting.

Practical use

The first commercial LEDs were commonly used as replacements for incandescent indicators, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches (see list of signal applications). These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Later, other colors became widely available and also appeared in appliances and equipment. As the LED materials technology became more advanced, the light output was increased, while maintaining the efficiency and the reliability to an acceptable level. The invention and development of the high power white light LED led to use for illumination (see list of illumination applications).
Most LEDs were made in the very common 5 mm T1¾ and 3 mm T1 packages, but with increasing power output, it has become increasingly necessary to shed excess heat in order to maintain reliability, so more complex packages have been adapted for efficient heat dissipation. Packages for state-of-the-art high power LEDs bear little resemblance to early LEDs.

Advantages

• Efficiency: LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving devices.
• Color: LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
• Size: LEDs can be very small (smaller than 2 mm2) and are easily populated onto printed circuit boards.
• On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds. LEDs used in communications devices can have even faster response times.
• Cycling: LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
• Dimming: LEDs can very easily be dimmed either by Pulse-width modulation or lowering the forward current.
• Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
• Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
• Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000–2,000 hours.
• Shock resistance: LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.
• Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
• Toxicity: LEDs do not contain mercury, unlike fluorescent lamps.