First, I would like to recall what a Joomla module is. Basically your Joomla page can be divided into different sections:
The main content is provided by the Joomla component and can be a single article or a blog view which is often used on the frontpage.
Everything else (typically smaller elements on your site) such as menus, small login forms, elements in the sidebar, breadcrumbs etc. are Joomla modules.
Based on my article about the development of a custom Joomla component I would like to add a module that displays smartphones stored in the database as a Joomla module.