Conventional Storage Water Heaters are the most popular type of water heater.
A Single Family Conventional Water Heater stores between 20 to 80 gallons of hot water. The way they work is the Hot water is released from the top of the water heater when you turn on your water tap. The water heater fills up from the bottom everytime the water heater gets low to ensure that there is always water stored in the tank for future use.
Conventional Water Heaters utilize a few heating sources such as Natural gas, Propane, Electricity and Fuel Oil. Propane and Natural Gas heaters work very similar. There is a burner under the water heater that heats up the water when the temperature gets lower than your desired temperature. Oil water heaters work similar but the oil is mixed with mist which is ignited by an electric spark near the burner on the water heater.
Because the water heaters are constantly being filled with water, a lot of energy can be wasted by having the water sit in the tank for a long period of time since it's constantly being heated to maintain the temperature. The one fix is to look for a Conventional water heater that has a significant amount of insulation.