Deleting a category will permanently remove it and any associated details. By deleting a category, the individual videos within the category will not be deleted—they will continue to appear in the list of videos; however, if they don't have another category, they will be removed from your catalog. If you need to delete videos, follow these instructions.
Navigate to the category you want to delete under Content > Organize > Categories and click Edit. Then scroll down and click Delete Category.