Hot water heaters come is many sizes, brands and styles but there are basically (3) types of hot water heaters.
First, there is the stand alone or free standing hot water heater. Pictured is various sizes of the free standing hot water heaters.
The top of the unit has your electrical line for power to the unit. Two water lines running into (cold) and out of (hot) the unit to supply hot water to your home.
There may also be oil lines or black iron gas pipe running to the hot water heater to the unit. Depending on what type of fuel operates the heater.
The second type of hot water heater is the indirect heater. You must have a FHW heating system to have this style of hot water heater. It is a tank that stores your hot water and is attached to your boiler. It is also fueled either by gas or oil, but your boiler or forced hot water heating system has the oil or gas lines attached, not the hot water tank itself.
The indirect heater has only the water lines coming from the heating system to the indirect tank and then a water line from the tank to supply your house with hot water.
Finally, the third is the tankless hot water heater. You do not see a water tank and the water is supplied by your boiler internally. Again, either natural gas or oil fired piping is attached to the boiler. You will have water lines from the boiler to supply your home with the hot water.