Solar Thermal really just means anything to do with capturing heat from the sun.  Passive solar thermal requires no moving parts, such as passive solar building design where the sun heats up the air inside your home or office.  Although these systems can be implemented on a large scale for electricity production, when considering active solar thermal at the home or office, systems are usually designed to meet domestic hot water demands.

Solar thermal is the most direct way to use solar energy—as well as the most cost effective.  The three different types of domestic solar thermal panels are easily added to existing structures.

Evacuated tube collectors use concentric reflective surfaces to concentrate solar rays on a tube of heat transfer fluid.  These systems net the hottest water, but are also the most expensive of the three types.  BRESCO LTD is a dealer in Apricus evacuated tube collectors.

Flat plate collectors capture heat from the sun directly and use a flat backing to transfer heat via conduction.  Flat plate collectors are easily recognized by the sheet of glazing that traps heat by utilizing the "greenhouse effect."

Seasonal pool collectors are ideal for spring and fall use to extend the swimming season.  Because these collectors are usually made from non-rigid PVC, they cannot withstand the temperatures that other collectors can.  However, they are significantly cheaper and easily stored when not in use.

Here is more information on Solar Thermal Systems from the North Carolina Sustainable Energy Association.