Wikipedia has a nice page on heat pumps which is compact and very informative.

Warmairc.com This site does a good job of describing all aspects of HVAC systems, equipment and practices in a short and easy to understand manner.

Additional Details

Equipment Images

 Air conditioners and Air source  heat pumps look identical, other than a heat pump should be elevated to facilitate drainage during defrost cycles.

Above is a 2 ton heat pump elevated on 6" risers