Direct principle. Water flows directly through the system and is heated without any other intermediate. It is very efficient, but what happens, for example in the highlands, where the temperature can be below the freezing point at night, water freezes and the pipes maybe break. In these cases, indirect solar water heater system are used.
Indirect principle. The liquid flowing through the panels is treated similarly to the water in the car radiators with antifreeze. Non-potable and poisonous for consumption, this requires separating the liquid that passes through the panels of the drinking water with a heat ex-changer, usually inside the water tank.