Understanding passing by reference

The difference between passing by reference and passing by value is a complicated and abstract concept in programming. It is one faced by many, many computer science students and baffles them to start with. Here is a simple visual explanation of the difference, how it works, and some examples in different programming languages. If you do not see your language of choice, feel free to suggest an example, or a request for the creation of one. Note that not every language directly supports passing by reference, but there are similar ways to achieve the affect if it is desired.