Energy Efficiency basically means to do more with less. The goal is to do the same thing (produce light, run a refrigerator, etc.) as before while using less energy. It is the most readily accessible and cost effective form of sustainability.
A larger investment in energy efficient design strategies usually means you will be spending less in the future. Energy efficient systems can pay for themselves monetarily and have the benefit of environmental rewards as well. There are products that pay for the increased cost of efficient design within a year and often the home is a more comfortable and enjoyable place to live.