There are two main types of solar thermal panels:  

Flat-plate collectors: These look similar to solar PV panels and are made up of a dark absorbing surface, a transparent cover, and a heat-insulating backing.

A fluid, usually a mixture of water and antifreeze, flows through the panel, absorbing the heat and transferring it to the storage tank.

Evacuated tube collectors: These consist of a series of glass tubes, each containing a heat-absorbing surface and a fluid-filled pipe.

The tubes are evacuated, creating a vacuum that helps to maintain high temperatures. Evacuated tube collectors are generally more efficient than flat-plate collectors, especially in colder climates.