CONTENTS

212 DUMITRU RĂDOIU(1) AND CĂLIN ENĂCHESCU(2)
the returned hits. Most of the hits were useless and extracting structured information
was the job of the humans. Developing an ontology for a domain and writing content
which adheres to it allows automatic extraction of new knowledge from different
web resources either to make it available for further automatic processing or to save
humans effort in finding high value structured information. Building an ontology of
a domain is not a goal in itself; it is just a means to an end.
The first benefit is that we share a common understanding of the way the information
in a domain is structured [3]. The second (with huge implications) is that
we can now separate the process of structuring the information of a domain from
the process of automatically retrieving new structured information from different web
resources.
2. Defining Ontology
”An ontology is an explicit representation of a shared understanding of the important
concepts in some domain of interest” [5]. Human knowledge is subjective.
Ontology is conceived to explicitly describe a cognitive structure upon which individuals
”objectivity agree about subjectivity” [6].
An ontology can be expressed at different levels of abstractions:
• in a natural language, as sets of declarative statements
• in a visual modeling language (e.g. UML) as a set of glyphs syntactically and
semantically organized
• in a formal language (e.g. OWL) as a set of formal statements
Natural language statements are difficult to process by a computer (program). Visual
representations - although very expressive - don’t provide enough flexibility. Hence
most ontology editors allow visual representations which they convert automatically
in a formal language.
So, the aim of any ontology is to capture the intrinsic conceptual structure of
the domain by providing a ”catalog” of the type of ”things” assumed to exist in
the domain, their properties and how can we ”reason” about them. It follows that
ontology is concerned with:
• The type of ”things” in the domain. ”Things” are called entities (objects,
actors), they may or may not have properties (which may take or take not
values)
• The explicit hierarchical organization/categorization of entities in a domain
(named taxonomy)
• Possible relations among entities (semantic interconnections). They are described
in short sentences (named triples)
• What terms (and rules to combine them) we can use when documenting
knowledge about the domain (vocabulary)
• What inference laws and logic laws can we used when we automatically process
a resource which adheres to our ontology