A solar collector is designed to collect heat by absorbing sunlight.   Solar collectors are usually mounted to a roof and used in a solar thermal system to heat houses and buildings. 

Flat plate solar collectors consist of a dark plate energy absorber, a transparent cover that allows solar energy to pass through but reduces heat loss, a heat transport fluid (water, antifreeze, or air), and insulation.  The absorber consists of thin black absorber sheet made out of special polymers or metal which is backed by a coil of fluid tubing.  After the fluid is heated it is circulated into a storage tank or heat exchanger.  In cold climates a mixture of water and glycol is used as the exchange fluid to prevent freezing.  Flat plates can be turned off with the use of a drain back system in the summer to prevent overheating.

Evacuated tube collectors consist of multiple glass tubes housed in a box.  The glass tubes heat up absorbers and solar transfer fluid (water or antifreeze).  The round surface of the tube means the surface of the tube is always perpendicular to solar rays which increases efficiency.  The vacuum tube also reduces heat loss.  The individual evacuated tubes are plugged into a header on the top and bottom of the rack.  The space in between the tubes reduces wind loads.  This system is popular for hot water heaters in the U.S.  Most evacuated tube collectors are made in China.  They are commonly used in German solar thermal systems because of the limited solar resource.  They are not as popular in the U.S. because they cost 25% more than flat plate collectors and are prone to stagnation in hot climates.

Batch heaters are essentially a sheet metal box with a tank.  They are a very basic system still popular in hot climates such as Latin America.