What Is A Plugin?
A Plugin is custom coded software functions which extends WordPress abilities beyond the core installation.
WooCommerce is a WordPress plugin and integrate seamlessly with WooCommerce extensions.
What is an Extension?
Extensions can be described as plugins for plugins that ‘extend’ the functionality of a specific plugin or theme. The main difference between a WordPress Plugin and an extension is that a plug-in provides additional performance which does not modify the core functionality. While an extension or ‘add-on’ is made to modifying core functionality.
What is the difference between an API and a plugin?
The WordPress Application Programming Interface (API) has a whole set of application programming interfaces, Each API is a way to communicate with a component of WordPress behind the scenes to use of a given set of functionality.