There are two types of collectors readily available to homeowners: flat-plate and evacuated-tube.  Both are designed to let sunlight enter and to trap its heat inside. 

Flat-plate collectors are the most common. They're insulated, weather-proof boxes covered by transparent or translucent glazing.  Inside the collector box is a dark metal plate that absorbs solar energy attached to copper pipes that contain a heat transfer fluid.

Evacuated-tube collectors are made of parallel tubes connecting to a common header. Each tube consists of a transparent glass outer tube (the collector) and an inner metal tube (the absorber), which is covered with a special coating that absorbs solar energy but inhibits heat loss. The air is withdrawn ("evacuated") from inside the glass tube to form a vacuum, which reduces heat loss from the absorber.

A third type, batch collectors, serve as both storage and collector.