For Wikipedia's categorization projects, see Wikipedia:Categorization.
Categorization is the process in which ideas and objects are recognised and understood. Categorization is fundamental in decision making and in all kinds of interaction with the environment. There are, however, different ways of approaching categorization.
The Classical View
The classical Aristotelian view that claims that categories are discrete entities characterized by a set of properties which are shared by their members. These are assumed to establish the conditions which are both necessary and sufficient to capture meaning.
Since the research by Eleanor Rosch and George Lakoff in the 1970s, categorization can also be viewed as a the process of grouping things based on prototypes - the idea of necessary and sufficient conditions is rarely if ever met in categories of naturally occurring things. It has also been suggested that categorisation based on prototypes (see Prototype (linguistics)) is the basis for human development, and that this learning relies on learning about the world via embodiment.
A cognitive approach accepts the fact that natural categories tend to be fuzzy at their boundaries and inconsistent in the status of their constituent members.
Systems of categories are not objectively "out there" in the world but are rooted in people's experience. Conceptual categories are not identical for every speaker of a language.
Categories form part of a hierarchical structure when applied to such subjects as taxonomy in biological classification: higher level: life-form level, middle level: generic or genus level, and lower level: the species level. These can be distinguished by certain traits that put an item in its distinctive category. But even these can be arbitrary and are subject to revision.
Categories at the middle level are perceptually and conceptually the more salient. The generic level of a category tends to elicit the most responses and richest images and seems to be the psychologically basic level. Typical taxonomies in zoology for example exhibit categorisation at the embodied level, with similarities leading to formulation of "higher" categories, and differences leading to differentiation within categories.