The luminous efficacy of a light source (bulb) is defined as the ratio of the light output (lumens) to the energy input (watts). The efficacy is measured in lumens per watt (lm/W).
Higher is the luminous efficacy, more the bulb gives light for the same amount of input energy.
For example BAZZ recessed fixture CFL 400, has a luminous efficacy of 1600 lumens/23 watts = 69,5 l/w. For the same amount of lumens, less energy is consumed compared to a 100W incandescent bulb which is 4 to 5 times less efficient : 1600 lm/100 W = 16 lm/W.
Information from Natural Resources Canada - Lighting Reference Guide.