Water heaters are made of steel. The inside of a residential water heater is lined with a glass lining. Eventually the water will find its way through small voids in the glass and rust through the steel. For this reason, a sacrificial anode rod is part of a residential water heater as it will corrode before the water heater steel will. Once the anode rod is gone, the tank will begin to corrode. Changing an anode rod every few years may prolong the life of the water heater, depending on the water quality. It is common for drain valves to begin leaking also. If the heater is shut off while you are out of town, or for prolonged periods of time, then started back up, the heating and cooling can cause any of the threaded joints to begin leaking. These should be, mostly, relatively easy to repair.