Solar Thermal is a collector either Flat Plate or Evacuated Tube , which is placed in primarily a Southerly direction, that will capture as much solar radiation as it possible can during the day to absorb the solar radiation.
Most panels you see on a roof are Evacuated Tube as they are more efficient.
The tubes you see have a vacuum inside and the dark colouring inside the tubes is called an absorber. This transfers the energy to the inner tubes which contain a fluid which is heated to a very high temperature and is pumped though the coil in your cylinder. Here it transfers the heat to the cylinder (heating your tank of water) and returns back up to the collect or to get re-heated and do the same process again.
There is a huge variety in product quality which will impact on overall product efficiency. The difference can be made in researching the product you want to buy that best suits your requirements.