What is the difference of extension and module in opencart?

I am new to opencart, and I am confused about the terminology between extension and module in opencart, could anyone explain to me?

In theory an extension is something that extends what is already there and a module is a creation of a new element to work with what is already there. However, they are used interchangeably, meaning, this has now (OC3) been interpreted (in opencart filing system logic) in one way like extensions are all additions and modules are a section, like payments for example.

So the bottom line is you can not assume there is any difference.

Extension is any piece of software that extends the main framework. Module is a type of extension. That's it, plain and simple.