We have always been familiar with a lightbulbs wattage. To most of us this signified how bright the bulb was. We could purchase 40w, 60w, 100w and we pretty much knew what kind of bulb we were getting. This however was not strictly true. The wattage of a bulb (or lamp as it is called in the industry) pretty much actually denotes the power consumption of the bulb. A 100w bulb is brighter than a 60W because it uses 40w more power to operate. The 100w did not refer to a particular brightness. The light output of bulbs has always been measured in lumens (though you couldn't tell this as it was never really displayed anywhere on the bulb itself or the box). With the invention of low energy bulbs, the lumen output is now very important.
It can still be very confusing for someone to compare 2 similar bulbs (one being a 50w halogen and the other being a 5w led) and yet both have the same light output. The wattages indicate to us that the 5w bulb uses far less power (and costs you less on your energy bill). What we actually need to compare is the luminosity of both bulbs to determine how similar they are. If the 50w version is 400 lumens and the 5w version is 380 lumens then we know both are very similar in light output.
New regulations now stipulate that manufacturers must indicate a bulbs lumen output on the packaging so next time you go to buy a light bulb, take a moment to compare the lumens from your standard bulb to a low energy equivalent. It may just save you a small fortune on your energy bill.