Thesauri and classification systems are traditionally used in search and retrieval of content of documents (sources). One of their most important components is the generic chain of hierarchy of concepts (classes) in which the features of the concept of a more general meaning (a more comprehensive class) are 'inherited' by the more specific concepts (classes). In the wake of aspirations toward working out the semantic web there appeared the so-called ontologies which are made up of the generic chains of hierarchy of concepts (classes) and rules formulated according to a logic of first order linked up with them. A key task of these is to secure the aforementioned inheritance of the generic hierarchy and to enable the drawing of conclusions with it. Ontologies can indirectly be traced back, for one, to Aristotle's system of categories and, for another, to Ranganathan's multi-dimensional theory of classification, a stimulus to modern classification, and by virtue of the latter, to cultures of the Far East as well. Ontologies are employed in expert systems and knowledge bases in order to provide for an information retrieval more automated in terms of semantics. Their proposal for standardization was also compiled early in 2003 (OWL). In the concluding part of the study an example of application for thesauri and classification systems is presented.