From whatis.com:

Taxonomy (from Greek taxis meaning arrangement or division and nomos meaning law) is the science of classification according to a pre-determined system, with the resulting catalog used to provide a conceptual framework for discussion, analysis, or information retrieval. In theory, the development of a good taxonomy takes into account the importance of separating elements of a group (taxon) into subgroups (taxa) that are mutually exclusive, unambiguous, and taken together, include all possibilities. In practice, a good taxonomy should be simple, easy to remember, and easy to use.

One of the best known taxonomies is the one devised by the Swedish scientist, Carl Linnaeus, whose classification for biology is still widely used (with modifications). In Web portal design, taxonomies are often created to describe categories and subcategories of topics found on the Web site. The categorization of words on whatis.com is similar to any Web portal taxonomy.