Craft provides a complete toolkit for customizing its features and functionality – almost every aspect of Craft can be extended, hooked into, or completely replaced. If you know PHP, you can build anything you want with it.

Most customizations come in the form of a module or a plugin.

As Yii’s documentation puts it, modules are “self-contained software units that consist of models, views, controllers, and other supporting components”. In other words, modules extend the system in various ways, without needing to change any of the core system code.

Modules can be simple, serving a single purpose like providing a new Dashboard widget type, or they can be complex, introducing entirely new concepts to the system, like an e-commerce application.

Plugins are a Craft-specific concept, so you won’t find any mention of it in the Yii docs. They can do everything modules can do (plugins actually are modules, technically), and some other things that make them better for being publicly distributed:

• They can be installed/trialed/purchased from the Craft Plugin Store.
• They can make database changes when installed, updated, or uninstalled.
• They get their own settings page within the Settings section of the Control Panel.
• They can be enabled/disabled by an admin, without running any Composer commands.

If the thing you want to build would benefit from those features, make it a plugin. Otherwise, a module might be better.