The vision of the Semantic Web (Berners-Lee, et al., 2001) is to have on-line resources expressed in a machine understandable format so that they can be interpreted and used by software agents. The ultimate goal is to enable Web users to find, share, integrate and thus reason on distributed information or knowledge more effectively. In the Semantic Web, ontology is one of the key concepts where entities and relationships intended to be modeled within some domains are described or posited as a form of knowledge. “Concept” and “role” are the two common terms used to refer to the two modeling elements in ontologies. Concepts (also called classes) refer to the abstract definitions of entities within the domain, whereas roles express the relationships between entities. On-line resources annotated with the labels of concepts are generally termed as instances of these concepts. Figure 1 shows a simple ontology about the domain of weapons, where concepts (e.g., “Weapon” and “Missile”) are annotated using ellipses, “subclass” roles and other user defined roles (e.g., “HasRange”) are annotated using dashed and solid lines respectively, and individual values are annotated using rectangles (e.g., “Liquid Fuel” and “Solid Fuel”). Resources about missiles can then be described accordingly using instances of concepts defined in this ontology.