A multiton is almost exactly the same as a singleton, the one slight difference is a multiton can store multiple instances of itself. Contrary to a singleton, which can only store one instance of itself.
As the name sort of implies, a singleton is a class that can only be initialised once per request to an application. E.g once per HTTP request on a website.
Design patterns have been around for decades, they were first defined properly by the Gang of Four (GoF). Since being defined they haven’t changed that much, in fact many patterns haven’t changed at all. Each pattern is placed into a group, based on what it attempts to achieve. These group are, creational, structural, behavioural and concurrent.
There are also architectural patterns, these are similar to design patterns, but they have a broader scope, taking the entire system into account, where a design pattern is more specific to an area of code.
Since writing this article, I have rewritten it in much more detail.
Just like APIE states the 4 basic concepts of OOP, SOLID defines 5 OOP principles which you should always follow when writing code. The letters stand for Single responsibility, Open-closed, Liskov substitution, Interface segregation and Dependency inversion. As much as all of these principles help in achieving a maintainable code base, the Open-closed and Interface segregation principles aren’t as useful as the others in my opinion as they’re harder to implement effectively and sometimes don’t give much benefit. Regardless I will explain what you need to know about each of these.