Model View Controller
A simple example to the valuable concept of MVC with little code for a quicker understanding.
Using MVC, it's quite easy to implement additional features later on or even switch the frontend completely, such as another GUI-framework, or use the same code base for a command line application instead.
I will try to keep the example short and simple, although the equipped language tries to nullify these ambitions ;)
MVC is a great pattern. It's easy to learn and applicable for coding (traditional applications, web apps, games, [...]), administration, engineering in general (both digital and real), etc.
Have fun ^__^