The Watt only measures the power required to operate a Light Bulb, whereas the Lumen measures the light output which the bulb will provide you with. A Lumen rating describes the actual brightness of light which you will see coming from the bulb.
The Lumen has taken a more central position in way that the lighting industry label their lighting products since the 2010 ruling by the EU that the Lumen is the approved method off selling and advertising lighting products.