The lumen (lm) is the SI derived unit for light flux, which is the amount of light being received by a surface. A lumen is defined as the amount of light being emitted into 1 unit of solid angle (1/4π of the area of a sphere) by a light source with a luminous intensity of 1 candela. Therefore a source of 1 candela will be putting out a total of 4π lumens.
Light bulbs are often rated in lumens in order to communicate their brightness. For example, a 13 watt CFL light bulb and a 60 watt incandescent light bulb have the same brightness of 800 lumens, therefore the CFL bulb accomplishes the same goal while using less power.
Lumens can be thought of as the "amount" of light within a beam or angle given off by a source, however it doesn't explain how much light will arrive at any given distance from the source. The unit for this is lux, which is equal to 1 lumen per square meter. A full moon on a clear night is tremendously bright (high lumen value), however the lux received on the Earth is about 1/4 of a lumen per square meter. In comparison, a light bulb will have an illumination of approximately 50 lux, while direct sunlight can get up to 130,000 lux at the Earth's surface.