A category is a label placed on a content item in order to better facilitate search, filtering, or grouping. Categories are how we broadcast content topics, and link to similar content.
Categories are most often used to inform the content management system that two pieces of content are connected.
Categories are normally organized into a hierarchical tree, with parent categories and subcategories. A taxonomy is largely synonymous with a category tree. Categories differ from tags in that they are structured and pre-determined, while tags are free-form and ad hoc.