A module is a lightweight and flexible extension that is used for page rendering. They are used for small bits of the page that are generally less complex and are able to be seen across different components.
You can see many examples of modules in the standard Joomla! install: – menus – Latest News – Login form – and many more.
This tutorial will explain how to go about creating a simple Hello World module. Through this tutorial you will learn the basic file structure of a module. This basic structure can then be expanded to produce more elaborate modules.