Custom Modules are an almost essential part of your Joomla Site and this guide will take your through the processes of creating one

• What is a Module?
  Modules are small building blocks/elements that make up your site. They usage is generally to display small snippets of information, whether than be latest content items or a slideshow of sorts.
• What is a Custom Module?
  A custom module, as the name suggest, is a user created module. You create these modules yourself in the Joomla administrator and are stored in the database. Characteristics of a custom module are normally the general Joomla module options as well as a large white box in which you input your data.
• Creating a Custom Modules
  The process of creating a custom module is simple. The first step is to login to the Joomla administrator. Next, navigate to Modules -> Site Modules. You are now in the Module Manager, you should notice an array of graphical buttons in the top right area of the Manager. One of these is the New button, select this to create the Custom Module.
• Configuring a Custom Module
  A Custom Module generally follow the same principle of a normal module, read the General Module Setting tutorials located here. The settings described at the tutorial are probably, the only options you would be changing.
• What is the White Box?
  The white box is where you input your code to create the Custom Module. The code that you are allowed to place has to be HTML. If you need to use other languages such as PHP, you will need to use a 3rd party module from the Joomla Extensions Directory.

You should now have successfully created and configured a Custom Module for Joomla.