Energy conservation refers to reducing energy consumption by using less than one energy service. Energy conservation differs from the efficient use of energy, which refers to using less energy for a constant service. Driving less is an example of energy conservation. Driving the same amount with a higher mileage vehicle is an example of energy efficiency. Energy conservation and efficiency are energy reduction techniques. Conservation of energy is part of the concept of sufficiency.
Although energy conservation reduces energy services, it can result in increased environmental quality, national security, personal financial security, and greater savings. It is at the top of the sustainable energy hierarchy. It also reduces energy costs by preventing the depletion of future resources.
Conserving energy is not about making limited resources last as long as they can, that would mean that you are doing nothing more than prolonging a crisis until you finally run out of energy resources all together. Conservation is the process of reducing demand in a limited supply and allowing the supply to begin rebuilding. Many times the best way to do this is to replace the energy used with an alternative.
In the case of fossil fuels, conservation may also include finding new ways to tap Earth's supply so that the commonly used oil fields are not drained completely. This allows those fields to be replenished more. This is not a process that happens overnight when you are talking about replenishing the natural resources you are talking about relieving excess demand in supply in 100 years of time to allow nature to recover.