A running toilet waste money and water, however it is usually a very simple fix. This problem is usually caused by a faulty valve. When the toilet flushes, a flapper inside the tank is opened up, so the water an be removed and allowing clean water to enter the toilet. If the flapper, or valve, is not sealing up correctly it will slowly allow the water in the tank to drain out. Then it tries to replace this water by allowing water to run into the tank. This video will explain the process of replacing this valve.