Questions about developing custom modules
A custom module is a local Magento module to add or extend features for a particular store. Every module must at least contain a module definition file in
app/etc/modules and a configuration file in
app/code/local/[NAMESPACE]/[MODULE]/etc/config.xml (Magento 1) or
registration.php (Magento 2).
Details how to create a module in Magento 2 can be found in the Magento documentation.
For questions about developing extensions for the public, see extensions. Extensions usually consist of one or more modules (see also: What is the difference between an extension and a module)