Much information is found regarding the issue of ontology; for this reason, the definitions in the area of ontologies are taken from Ontology Development 101: A Guide to Creating Your First Ontology (Natalya & Deborah, 2005). It is official guidance provided by Stanford University and founder of Protege (Stanford Center for Biomedical Informatics Research, 2010), which is the most used tool in the creation of ontologies.
The Semantic Web literature contains several definitions of ontology. In addition, many definitions contradict others definitions. In order to standardize definitions in this chapter, an ontology is a formal and explicit concepts in a specific domain (classes [also called concepts]), properties of these concepts that contain various features and attributes (slots [also called roles or properties]). Finally, also have restrictions on the properties (facets [also called role restrictions]). An ontology together with a group of individuals of classes constitutes a knowledge base. Actually, there is a fine line where the ontology ends and the knowledge base begins. Classes are the focus of most ontologies. And the classes describe concepts in a domain (Natalya & Deborah, 2005).