Sometimes they get used interchangeably (eg. people might call "Logo Design" a category, it also might be called a subcategory). Though really I think the top level are the categories and the ones below are the subcategories.

eg. The Categories are:

Graphics & Design, Digital Marketing, Writing & Translation, Video & Animation, Music & Audio, Programming & Tech, etc.

and the subcategories are below them, eg:

Cartoons & Comics (Graphics & Design->Cartoons & Comics)

Logo Design (Graphics & Design->Logo Design)

Illustration (Graphics & Design->Illustration)

The ones below those subcategories could also be called subcategories (or nested ones).