The category.twig view is used to display the products within a particular category.
In most circumstances, the category page will extend the collection.twig template because it's format will follow the usual display of collection pages on your theme.
The category page has access to the category object to display details about the current category being viewed (e.g. the category title, description, image) and also the product object to display details about each product within the category.
In most circumstances, the category page will include the items partial to display the details of products.