Light Emitting Diode
A Light Emitting Diode (usually shortened to the acronym, LED) is a semiconductor device which uses quantum effects to turn electricity directly into light at very high efficiency (photon emission when an electron changes energy levels). The fundamental nature of LEDs is such that they are diodes, electrically, but this is usually immaterial to their use.
LEDs are rapidly becoming the preferred source of illumination, since not only are they extremely efficient, which means that they produce little heat (making them much easier to emplace), they are also quite robust (unlike incandescent filament lights, which are quite fragile when illuminated), and have extremely long lives - usually considerably longer than the devices they are built into.
LEDs produce light of a single colour (not quite a single wavelength, but close). This leads to another advantage of LEDs: for applications where a particular colour is needed, rather than generating white light and passing it through a filter (discarding the energy in the un-wanted colours), LEDs can generate light of the required colour directly.
To produce 'white' LEDs, two methods are used. The first was to use a blue LED which contains a phosphor which absorbs some of the blue light, and then fluoresces, producing yellow light; the combination of the yellow, and the remaining blue, seems white. It is not a 'good' white, though, and a later alternative used phosphors which produce red and green light, which together with the blue produce a better 'white'.
The first experimental LEDs were produced as early as the late 1920s, and experimentation continued through the 1950s (as understanding of semiconductors took off). The first commercial LEDs appeared in 1962, operating in the infra-red range; red followed shortly thereafter. They were still extremely expensive, though, and the price did not come down until 1968. Early units produced too little light for illumination, and they were only used as indicators.
Yellow and green LEDs followed shortly thereafter, but blue LEDs remained a difficult problem, requiring decades of work by a number of different parties before they were finally produced commercially in 1994. Work since then has been focused on devices which have more total light output; it is now clear that as the costs come down, LEDs will completely replace the Edison (filament) light.