Here is my water heater.   The line from top left to bottom right and continuing on is cold water.  The one starting top right to bottom left is hot water.

The small line is the propane feed.

Outside view.  The water heater is available in many configurations.  

Mine is the cheapest pilot light type heater.  There are model with DSI ignition, electric heating elements, and connections to hook up another heat source like a solar panels.

This one is a six gallon unit.  I have not tested it.