Except for the framework code, all of the code bases of Odoo are packed in the form of modules. These modules can be installed or uninstalled at any time from the database. There are two main purposes for these modules. Either you can add new apps/business logic, or you can modify an existing application. Put simply, in Odoo, everything starts and ends with modules.

Odoo is being used by companies of all sizes; each company has a different business flow and requirements. To deal with this issue, Odoo splits the features of the application into different modules. These modules can be loaded in the database on demand. Basically, the user can enable/disable these feature at any time. Consequently, the same software can be adjusted for different requirements. Check out the following screenshot of Odoo modules; the first module in the column is the main application and others are designed for adding extra features in that app. To get a modules list grouped by the apps category, go to the Apps menu and apply grouping on Category:

If you plan on developing the new application in Odoo, you should create boundaries for various features. This will be very helpful for dividing your application into different add-on modules. Now that you know the purpose of the add-on module in Odoo, we can start building our own add-on module.