I never knew....................

Information Science Glossary

The big four:

taxonomy

A grouping of terms representing topics or subject categories. A taxonomy is typically structured so that its terms exhibit hierarchical relationships to one another, between broader and narrower concepts. Taxonomy structure is discussed in the NISO Z39.19 (2005) standard.

thesaurus

A thesaurus puts terms into context by defining a variety of relationships among the thesaurus terms. As with most taxonomies, thesauri define broader and narrower term relationships (hierarchical relationships). In addition, they specify related terms (associative relationships) that allow the user to identify conceptual relationships between terms in different term groupings. One more type of term relationship in thesauri is the synonym relationship or equivalence relationship, which establishes preferred and non-preferred terms. Scope notes can be attached to any term to clarify the specific meaning of the term within the thesaurus. These elements – the hierarchical, associative, and equivalence relationships and scope notes – work together to enrich a thesaurus and make it far more than a simple word list.

Thesaurus structure is dictated by national and international standards, 

such as ANSI/NISO Z39.19 and ISO 2788.

classification

Classification is the backbone of organizing fields of knowledge and indexing. Its physical counterpart is the placing of a book or journal in a single spot on a library shelf. In a computer environment, a single object can be classified in several locations with a polyhierarchical system. At the core of classification is content analysis.

ontology

Ontology is a form of classification that goes beyond the three types of term relationships described above, to indicate specific functional relationships among terms. Whereas in a taxonomy terms may be classified together as “fruit”, in an ontology the conceptual relationship may define a fruit term in different contexts such as “fruit which is used in pies”; the ontological relationship might be that the item represented by term A is "used in" the item represented by term B. Ontological relationships are inherently self-describing. Ontologies are the backbone of the semantic web, as they provide multiple links to data and therefore can support search and insightful navigation.

semantics

The study of meaning in language.

http://www.dataharmony.com/library/taxonomyGlossary.html