If you have an electric water heater it will use less electricity when you are away than if you were home using hot water. If an electric water heater is left energized during your vacation it will continue to maintain the tank temperature, even if you're not using hot water.
Heat is lost through the insulation and copper pipes that come out the top.
If you are going to be away for more than a few days, you might want to consider turning your water heater off at the panel box. If the temperature drops while you are way your furnace or electric heat will still run to maintain the temperature set on your thermostat. Setting the thermostat lower may save you money, but this might not always be the case. Be careful not to set the thermostat so low that a potential for freezing could occur in your home. If refrigerators and freezers are not emptied and turned off while you are away they will continue to operate in order to maintain preset temperatures.