A measure of illuminanace, or the amount of light that falls on an area. The light is measured in lumens and the area in square meters, so the units are lumens per square meters (lm/m2.) You measure lux with a lux meter; I use a Dr. Meter Digital Illuminance/Light Meter LX1330B which can be had for about $40.
Both LED components and fluorescent lamps make use of phosphors. These phosphors are a chemical blend, which when excited with a single wavelength of lights (blue in the case of LEDs; UV in the case of Fluorescents) emits all different colors of visible light, which the human eye perceives as white light. Usually yellow in appearance, you can see the phosphor in the photo of the mid-power LED component above.
Because LEDs are such an intense single-point light source, lamps without diffusers allow the LEDs to appears as intense dots rather than a smooth even glow. This point-source look is called pixellation.