Traditionally it was never a problem to buy a light bulb in the past. You had a clear perception of how bright a 25w or 40w or a 100w would be. The true measure of brightness is actually Lumens and not Watts. The higher the lumens, the brighter the bulb. Watts is the measurement of how much electricity the bulb uses. The higher the wattage, the higher your electricity bill for a similar usage.
With newer types of bulbs, it takes far fewer watts to create just as much light, so wattage ratings are no longer very useful. Each type of bulb is different, and the whole idea is to develop bulbs that use fewer watts to make more light.
Lumens actually measure the amount of light being put out by the bulb. Lumens are a much more accurate measurement, because it tells you how the light actually performs, regardless of the source that produced it.