Lighting is one of the major purposes for which electricity is generated worldwide. As such, replacing incandescent bulbs (which produce a lot of unnecessary heat along with light) with more efficient options is one mechanism through which electricity demand can be controlled and emissions of greenhouse gas pollution kept in check.
LEDs are a solid state lighting option that is even more efficient than compact fluorescent bulbs. Up to now, LEDs have had a much higher up-front cost than either incandescents or LEDs, but the technology is evolving and costs are coming down.
Here are a couple of recent articles on the technology: