* <-- Smoke
              *           *
    Bowl --> \_*_/       *  *
              | |        **
     Pipe --> |*|       // <-- Mouthpiece
              | |      //
Carb --> 0   *| | *  */
         |* * | |**  |
         |~~~~|*|~~~~| <-- Waterline
         | *  | | ** |
         |  *  ** *  |

The above is a simple diagram of what a bong may look like and the description below is how it works. There are many types and variations of bongs which vary in complicatedness, but this one here has been kept basic for simplicity's sake.

How it works: Preferred smoking substance is placed in the bowl (the bottom of the bowl contains a fine-meshed screen so air and smoke can travel down but the material cannot fall in) and it is lit so it starts to smolder. Then, the carb is covered (the carb is just a hole to let air in and out for regulating the vacuum) and the air already inside of the container is sucked out through the mouthpiece to create a vacuum. If the vacuum is kept, the surrounding air on the outside of the chamber will naturally want to migrate into it to equalize the pressure, and the only path for it to do this is through the bowl, pipe, and water. As the air rushes through the bowl it pulls smoke along with it from the smoldering substance down the pipe and then bubbles up through the water into the vacuum area. Once it contains enough smoke for a hit, the carb is uncovered and the person sucks the air and smoke through the mouthpiece for inhalation. Once you have uncovered the carb, the vacuum is broken and any suction on the mouthpiece will draw the air and smoke from the chamber, but not from the bowl. All of these basic principals of vacuums, high-pressure, and low-pressure might have seemed boring to learn in grade school, but now you can have some fun with them.