What is energy efficiency?
Energy efficiency is "using less energy to provide the same service".
There are other definitions, but this is a good operational one.
The best way to understand this idea is through examples:
When you replace a single pane window in your house with an energy-efficient one, the new window prevents heat from escaping in the winter, so you save energy by using your furnace or electric heater less while still staying comfortable. In the summer, efficient windows keep the heat out, so the air conditioner does not run as often and you save electricity.
When you replace an appliance, such as a refrigerator or clothes washer, or office equipment, such as a computer or printer, with a more energy-efficient model, the new equipment provides the same service, but uses less energy. This saves you money on your energy bill, and reduces the amount of greenhouse gases going into the atmosphere.
Energy efficiency is not energy conservation.
Energy conservation is reducing or going without a service to save energy.
For example: Turning off a light is energy conservation. Replacing an incandescent lamp with a compact fluorescent lamp (which uses much less energy to produce the same amount of light) is energy efficiency.
Both efficiency and conservation can reduce greenhouse gas emissions.