Ontologies are formalism whose purpose is to support humans or machines to
share some common knowledge in a structured way. They allow the concepts and
terms relevant to a given domain to be identified and defined in an unambiguous
way. As such, ontologies are seen as the key technology used to describe the semantics
of information at various sites, overcoming the problem of implicit and
hidden knowledge and thus enabling exchange of semantic contents. In this report
we survey the work on ontologies that has been carried out in recent years. In
particular, we first overview the languages that have been proposed for representing
ontologies, and present the work on reasoning over ontologies. We then overview the
work on ontologies from four different points of view: (i) We survey methodologies
for designing and maintaining ontologies, presenting automated tools suitable for
such tasks. (ii) We present languages and architectures for accessing, processing
and in general making use of ontologies. (iii) We presents several approaches for
integrating and merging ontologies by detecting correspondences among them. (iv)
Finally, we present different approaches for making heterogeneous and autonomous
ontologies interoperate, in the sense that the various ontologies are not modified as
an effect of interoperating with the others.