Lumens defines the intensity of light emitted.
For decades, watts have been used to indicate the relative brightness of lights. This was a satisfactory approach during the time when incandescent lights represented the vast majority of light sources. However, watts is a measure of power used, not brightness; hence with the advent of new, more efficient lighting such as CFLs and LEDs, using watts to define brightness was no longer relevant.
For reference purposes, a 100-watt incandescent bulb produces approximately 1600 lumens of emitted light. The latest LEDs can produce the same level of light output using less than 20 watts.
View more technical description of lumens
The following chart shows lumens and power used for the three primary light sources: