A philosophical and technical approach of the concept of ontology is introduced and the importance of ontologies to structure, manage and retrieve the knowledge of different scientific fields and various political and social contexts is enhanced. The tools related to ontology management and the languages used for ontology creation are revised. In particular, an identification of the web-based ontology mark-up languages that contribute to knowledge representation and organization in this electronic and hypertextual environment, such as DAML+OIL, OWL and OML, is carried out.