One of the most powerful Joomla features is the ability to override component and module views.
About Module Layout Overrides
Modules, like components, are set up in a particular directory structure.
default.php (the layout)
helper.php (a helper file containing data logic)
mod_latest_news.php (the main module file)
mod_latest_news.xml (the installation XML file)
Similar to components, under the main module directory (in the example, mod_latest_news) there is a /tmpl/ directory. There is usually only one layout file but depending on who wrote the module, and how it is written, there could be more.
As for components, the layout override for a module must be placed in particular way. Using Beez as an example again, you will see the following structure:
/mod_latest_news (this directory matches the module directory name)
default.php (this file matches the layout file name)
The structure for module overrides is again quite simple: /html/mod_module_name/layout_file_name.php.
Copying or Creating Layout Files
The rhuk_milkyway template does not have any layout overrides for any modules. If we want to override the default layout for Latest News module, we need to copy this file:
to this location, creating the approriate directories in the event they don't already exist:
You need to take a little care with overriding module layout because there are a number of different ways that modules can or have been designed so you need to treat each one individually.
Here you can find the full explanation about overrides:
Understanding Output Overrides