Water heating is a thermodynamic process, meaning that any type of water heater you use requires thermal energy to work. In today's modern home, with ever-advancing technologies and choices, the question is more about what drives the water heating process. Conventionally, water heaters have been powered by gas or electricity.
Water heaters that use direct thermal energy (sans electricity) to heat water include: gas (compressed natural gas, propane, butane); solar; oil; and sometimes geothermal. In some cases, waste heat, or steam, from a space heater is used to heat potable water for the home, although this is much more common in Europe and commercial-sized systems in the U.S.
Solar and geothermal are two thermal water heaters gaining traction today. By far, solar dominates among renewable water heating options. Geothermal systems are increasingly used for space heating and can be modified to heat water as well. Aside from electric water heaters, natural gas heaters are the most common. They use a burner at the bottom to heat water in a tank. Below are more detailed examples of for thermal water heating and the different types of systems for each.
Thermal Water Heater