Solar thermal collectors capture and retain heat from the sun and transfer this heat to a liquid. The simplest approach to solar heating of water is to simply mount a metal tank filled with water in a sunny place. The heat from the sun would then heat the metal tank and the water inside. Indeed, this was how the very first SWH systems worked more than a century ago. The challenge is however, to limit the heat loss from the tank, thus delaying the time until thermal equilibrium is reached between the water inside the tank and the air.
Evacuated Tubes Collectors:

Evacuated Tubes solar thermal collector
Evacuated tube collectors (ETC) are a way in which heat loss to the environment, inherent in flat plates, has been reduced. Since heat loss due to convection cannot cross a vacuum, it forms an efficient isolation mechanism to keep heat inside the collector pipes. Since two flat sheets of glass are normally not strong enough to withstand a vacuum, the vacuum is rather created between two concentric tubes. Typically, the water piping in an ETC is therefore surrounded by two concentric tubes of glass with a vacuum in between that admits heat from the sun (to heat the pipe) but which limits heat loss back to the environment. The inner tube is coated with a thermal absorbent. Life of the vacuum varies from collector to collector, anywhere from 5 years to 15 years.

Flat Plate Collectors:

Flat-plate solar thermal collector
Flat plate collectors are an extension of the basic idea to place a collector in an ‘oven’-like box with glass in the direction of the Sun. Most flat plate collectors have two horizontal pipes at the top and bottom, called headers, and many smaller vertical pipes connecting them, called risers. The risers are welded (or similarly connected) to thin absorber fins. Heat-transfer fluid (water or water/antifreeze mix) is pumped from the hot water geyser (direct system) or heat exchanger (indirect system) into the collectors’ bottom header, and it travels up the risers, collecting heat from the absorber fins, and then exits the collector out of the top header. Serpentine flat plate collectors differ slightly from this “harp” design, and instead use a single pipe that travels up and down the collector. However, since the water will not be able to rise to the top of an s-pipe, serpentine flat plate collectors can only be used in pumped systems. The type of glass used in flat plate collectors is almost always low-iron, tempered glass. Being tempered, the glass can withstand significant hail without breaking, which is one of the reasons that flat-plate collectors are considered the most durable collector type.
Flat plate collectors are generally the same as ETC in full sunshine conditions. However, the energy output of flat plate collectors is reduced slightly more than evacuated tube collectors in cloudy or extremely cold conditions.
Unglazed/Formed Collectors:
Unglazed or formed collectors are similar to flat-plate collectors, except they are not thermally insulated nor physically protected by a glass panel. Consequently these types of collectors are much less efficient for domestic water heating. For pool heating applications, however, the water being heated is often colder than the ambient roof temperature, at which point the lack of thermal insulation allows additional heat to be drawn from the surrounding environment.