nice read:
Let's start with a normal electric light bulb like you see in any normal household lamp. A normal light bulb is made up of a fairly large, thin, frosted glass envelope. Inside the glass is a gas such as argon and/or nitrogen. At the center of the lamp is a tungsten filament. Electricity heats this filament up to about 4,500 degrees F (2,500 degrees Celsius). Just like any hot metal, the tungsten gets "white hot" at that heat and emits a great deal of visible light in a process called incandescence. See How Gas Lanterns Work for more information on incandescence.
A normal light bulb is not very efficient, and it only lasts about 750 to 1,000 hours in normal use. It's not very efficient because, in the process of radiating light, it also radiates a huge amount of infrared heat -- far more heat than light. Since the purpose of a light bulb is to generate light, the heat is wasted energy. It doesn't last very long because the tungsten in the filament evaporates and deposits on the glass. Eventually, a thin spot in the filament causes the filament to break, and the bulb "burns out."
A halogen lamp also uses a tungsten filament, but it is encased inside a much smaller quartz envelope. Because the envelope is so close to the filament, it would melt if it were made from glass. The gas inside the envelope is also different -- it consists of a gas from the halogen group. These gases have a very interesting property: They combine with tungsten vapor! If the temperature is high enough, the halogen gas will combine with tungsten atoms as they evaporate and redeposit them on the filament. This recycling process lets the filament last a lot longer. In addition, it is now possible to run the filament hotter, meaning you get more light per unit of energy. You still get a lot of heat, though; and because the quartz envelope is so close to the filament, it is EXTREMELY hot compared to a normal light bulb.
LEDs work on a completely different principle to ordinary lamps, and it's a bit difficult to explain fully without a little delving into semiconductor physics, so here goes! Basically, electricity is made up of the flow of electrons through a conductor under the influence of an externally applied voltage, the overall flow made up of individual electrons jumping from one atom to another. In good conductors like metals, these individual jumps don't involve a lot of energy, while in insulators the electrons are very tightly bound. However, in semiconductors like silicon and germanium, individual electrons are free to move but the jumps involve much higher energy levels, dictated by a property known as the "band-gap" - it's easiest to describe this as an indication of the electrical pressure needed to dislodge electrons from the parent semiconductor atoms. This is how the term "semiconductor" originated, their electrical properties are somewhere between conductors and insulators, and for what it's worth both silicon and germanium are known as "metalloids" with both physical and chemical properties between those of metals and non-metals. In one particular family of compound semiconductor materials based on the rare metal gallium, (compounds of gallium with arsenic, indium, phosphorous, aluminium and nitrogen) the band-gap is so wide that appreciable energy is needed to make electrons jump. When each electron recombines with an atom, it emits a particle of light - a photon. These jumps take place across the junction between two regions of the semiconductor crystal - one formed with an excess of electrons (n-type), the other with a corresponding deficit (p-type). Semiconductor materials based on gallium have the useful property of being transparent, allowing the light generated to escape from the junction.
The colour of light is determined by its frequency, which in turn depends on the energy expended to generate each photon. This can be altered by slightly adjusting the band-gap of the semiconductor material and different colour LEDs are made by varying the "recipe" of elements in the semiconductor crystal. LEDs are now available covering the entire spectrum from the near infra-red (used in TV remote controls) to the newly-developed ultra-violet types. LED light is mostly monochromatic - it is made up of one single colour only.