The Web ontology
language (OWL) adds to RDF tools for defining more sophisticated semantic
constructs (classes, relationships, constraints) still using the RDF-constrained XML
syntax. Computers can be programmed to parse the XML syntax, find RDF-encoded
semantic relationships, and resolve meanings by looking for equivalence relationships
as defined by OWL-based vocabularies or ontologies.
Ontologies are virtual dictionaries that formally define the meanings of relevant
concepts. Ontologies may be foundational (general), or domain-specific and are
often specified hierarchically, relating concepts to one another via their attributes.
As ontologies emerge across the Semantic Web, many will overlap, and different
terms will come to define any given concept. Semantic maps will be built to relate
the same concepts defined differently from one ontology to another (Doan, Madhavan,
Domingos, & Halevy, 2002). Software programs called intelligent agents will
be built to navigate the Semantic Web, searching not only for keywords or phrases,
but also for concepts semantically encoded into Web documents (Berners-Lee,
Hendler, & Lassila, 2001).
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