What is a module?
Modules are small programs that can be inserted anywhere in a site that uses Joomla.
Depending on the template (the part that handles the entire display) can be placed anywhere dynamically configuring it on the backend.
The function of the modules is basically display information in a small box, for example, could be used to show user data, date and time, temperature of a place in particular, etc. You can also use a module to link it to component and use it as a submenu.
You need 2 files to create a module: One is the XML file that is used for installing and configuring the module in Joomla. The other is the PHP file itself, which perform the task of displaying data on the site. Is necessary to respect the nomenclature putting “mod_” to the name of the module. This is important in order to install it on the Joomla´s backend.