Ontology Semantics and Applications Panorea Gaitanou Benaki Museum. Library, Athens, Greece gaitanou@benaki.gr Abstract. This paper presents the widespread use of ontologies in many areas. One of the most important is Semantic Web development. The existence of numerous search engines, which produce results simply by using keywords, is an ineffective method of finding the requested information. With the use of ontologies, intelligent search could be performed, based on semantics and concepts. Furthermore, in the e-commerce field, it is explicitly necessary to integrate and combine applications from different enterprises and indefinable single web entities. Moreover, in the multimedia and graphics field, ontologies can be expanded from their original form and each concept can be described as a multimedia entity. Multimedia ontologies could help in content indexing and optimization, in knowledge sharing and learning based applications and can provide reasoning services. In Peer-to-peer networks and grid technologies, ontologies can facilitate the interoperability among computer systems. Finally, in Pervasive (or Ubiquitous) Computing Environments, ontologies can address several issues concerning the development and management of such systems. Keywords: knowledge management, ontologies, grid technologies, pervasive computing, e-commerce, multimedia 1. Introduction In recent years, ontologies have moved from the AI (Artificial Intelligence) research community into real-word applications in a number of domains. In many disciplines, standardized ontologies have been developed, so that domain experts can use them to share and annotate information in their fields. Ontology is defined as a formal explicit specification of a shared conceptualization [18]. In other words, it is a shared understanding of some domain interest, which is often realized as a set of classes (concepts), relations, functions, axioms and instances. Fensel, in an attempt to analyze the complex definition given by Gruber, emphasizes on the four basic components: “conceptualization” is an abstract model of a phenomenon in the world; an ontology is “formal” because it should be machine readable and “explicit” because the concepts used and the constraints on their use are explicitly defined; finally it is “shared” because there is an agreement between those who use the ontologies [19]. Ontologies try to capture the semantics of domain