There are a number of different types of solar water heating systems, including either thermal or hydronic. Thermal systems use the heat from the sun to directly warm water, while hydronic systems transfer that heat through pipes networked throughout your home in order to evenly distribute it.
Either system is typically less expensive and more efficient than traditional fossil-fuel-based water heating methods like gas or oil furnaces. Solar panels can also be mounted on your roof to generate even more electricity for your system.