Any machine that you use in your home that needs water, such as doing laundry to washing dishes or taking a hot shower, requires a water heater.
The most common type of water heater is one that stores water in its tank at a desired temperature and is available whenever you turn it on.
A gas water heater pulls cold water into the tank with a dip tube. The gas burner then heats the water. When the burner burns gas, it pulls really hot and toxic water through a chimney that is in the middle of the tank.
The chimney pulls the toxic air outside while heating the metal chimney which then heats the water around it. When you turn on a faucet, the water heater then moves the heated water to all the plumbing in the house.
The heated water rises, and as cold water is brought back in through the dip tube, the hot water is pushed out. You can set the temperature of your water with the thermometer that is connected to the gas line.
If the water temperature gets too high or if pressure is building up in your take, there is a valve that opens up to release the water.
This is the temperature and pressure relief valve. The helps protect you and made sure your water heater does not explode. It also has a drain valve on the water heater to drain it and reduce the amount of sediment that builds up in the tank. This drainage should be done annually.