Essential concepts

You're probably just getting started with Odoo, so now is obviously a good time to explain Odoo modules and how they are used in an Odoo development.

Understanding applications and modules

It's common to hear about Odoo modules and applications. But what exactly is the difference between them?

Module addons are the building blocks for Odoo applications. A module can add new features to Odoo, or modify existing ones. It is a directory containing a manifest, or descriptor file, named __manifest__.py, plus the remaining files that implement its features. 

Applications are the way major features are added to Odoo. They provide the core elements for a functional area, such as Accounting or HR, based on which additional addon modules modify or extend features. Because of this, they are highlighted in the Odoo Apps menu.

If your module is complex, and adds new or major functionality to Odoo, you might consider creating it as an application. If your module just makes changes to existing...