There is a whole lot of misunderstanding of object-oriented programming, especially in the WordPress universe. Proper OOP utilizes various concepts around classes and objects, and the relationship between those. A fundamental aspect of OOP is being able to re-use code. Also, for every kind of problem you have to solve, one or even several solutions might exist already.

Design patterns are such things that you can leverage in different situations and contexts, and that allow for reuse of (parts of) your code. So what are design patterns, why and how can we make use of them, are there any WordPress-specific patterns, or ones that cannot really be used in a WordPress context? Given problem X, is there always a matching pattern? Could there be more? How do I know what is the best one?