Both solar air and solar water heaters convert sunlight to heat, and both can be divided into active systems, which operate with the help of pumps or fans, or passive systems, which do not.
Solar water heaters start with collectors. Water (or other fluid) is heated in the collector and then pumped to a storage tank to be used throughout a building. A collector normally consists of a tube and either a flat, dark absorber or a mirrored trough. The absorber and trough concentrate sunlight on the tube filled with water (or other fluid), which is then moved into a tank via pumps after it is heated. This water (or other fluid) can then be used in many ways, either in an open loop system in which the actual water is used to drink, cook, or clean, or in a closed loop system, in which the water (or other fluid) is used as a medium to transport heat energy.
Solar air heaters work in much the same way. A collector is used to heat the air, and then the air is circulated throughout a building. Many solar air heating systems are also known as thermosiphon systems. Thermosiphon systems do not use fans or pumps, but instead rely purely on natural convection for circulation. The hot air rises and is collected in a tank, and once it has cooled it falls back to the collector.
In both solar air and solar water heaters, the collectors are located on south-facing walls to maximize the sunlight hitting the collectors.