In Craft you have the possibility to create categorized structures to manage certain fields within the content. For example, it can be used for systematic filtering. A developer can use categories in the content structure as desired.
Main categories themselves can only be created by a site developer. Go to Categories to view any existing categories. Within a main category, you have the option to create new categories and subcategories. You can re-arrange them by moving them to a different order or category.
Set up categories can be used in Craft in a variety of ways. For example, on a product overview page where you want the customer give the ability to filter products by color. The visitor is looking for a red product, clicks Red and all products with the related color category are filtered and displayed on the screen.
While adding products in Craft, one or more categories can be selected.