There are basically three different types of thermal solar collectors: flat-plate, evacuated-tube and concentrating.

This consists of a insulated, weatherproof box containing a dark absorbing plate which is situated underneath a transparent cover. Water passes through pipes situated underneath the absorbing plate and as water passes through these pipes it is heated. Although the flat-plate collector is inferior to the evacuated tube collector, it is still one of the most common used.
Evacuated tube
The solar water heaters consist of glass tubes parrallel to each other which contain a special heat-transfer fluid to be heated by the sun. This fluid then flows up to heat the water in the cylinder, and will then flow back to the bottom of the collector to start the cycle again.
Concentrating collectors
These usually consist of a parabolic trough which contains mirrored surfaces which concentrate the sun's energy onto a collector containing either heat-transfer fluid or even water itself. This type of collector is seldom used as it is inefficient in comparison to the evacuated tube collectors.