LED stands for light emitting diode. You've probably got some on your keyboard - they are the ones which light up when you hit caps lock or scroll lock. They are solid state devices which are more shock resistant than incandescant bulbs and more efficient too (more light for given power). In recent times due to advancing technology it has become viable to use them as a light source in flashlights. They are becoming increasingly popular in flashlights because for the above two reasons and one other important advantage: beam colour. LEDs come in a variety of colours, the most popular of which is white. Compared to incandescant lights, white LEDs eg. lumileds luxeons or nichia jupiters produce a whiter colour. An analagous example would be comparing fluorescant light to an ordinary 100w bulb's light.