A type of aggregation which represents a conceptual structure to which content objects are assigned. The assignment is simple and binary – the object is assigned, or it is not. Objects can normally not be assigned in any particular order, nor can the assignment carry any additional information.
Categories are normally organized into a hierarchical tree, with parent categories and subcategories. A taxonomy is largely synonymous with a category tree.
Tags have the same functional mechanism of categories (a structure to which items are assigned), with some subtle differences:
Tags are normally “flat,” without hierarchy
Tags can often be created on-the-fly by editors while they organize content
The editorial interface is often just a simple textbox, sometimes with type-ahead detection
Note that categories are normally not managed content, but rather a separate system meant to organize content. In some systems, a category is really just a content object of a specific type to which other content objects are related. This blurs the traditional designation of a category.