Ram pumps

My father, who is a jack of all trades, and master of most of them, recently explained to me about ram pumps.  I think I have the general idea and I found it quite interesting.

The basic premise is to take a small creek or running water source and setup a series of pipes that will pump water uphill under it's own power.  It is based on the simple scientific principle of gravity.  It was suggested to me as a way to fill a small water tower or barrel for irrigation.  I looked up a site online that sells ram pumps and was amazed.  You can also build your own with a few supplies and some know-how.  I'd love for someone to offer a more detailed explanation of how this works.