What we call a light bulb, is referred to as a lamp by the lighting industry. Compact fluorescent lamps (CFLs) are low energy light bulbs. They use up to 80% less energy than old fashioned incandescent bulbs, but to do this, they need a small amount of mercury which is why they can’t be thrown away in your bin.
The typical amount of mercury in a florescent lamp is between 1.2 to 4mg, and in a domestic compact florescent lamp (CFL) this equates to the tip of a ballpoint pen.
The mercury in one CFL is not enough to pose a health risk but they do need to be disposed of responsibly to reduce the risk of large quantities of mercury ending up in landfill. In Europe the collection and recycling of fluorescent lamps is compulsory under the Waste Electrical and Electronic Equipment (WEEE) Directive
Compact LED bulbs can look very similar to CFLs, this means you can change to LEDs without having to change your light fittings.