## 4 RDF Graphs: Definitions, Concepts, Examples  
Since RDF graphs play a central role in our research, in this Section, we focus in a greater detail on definitions, concepts and algorithms of such very important data structures of the popular Semantic Web. RDF is a Semantic Web data model oriented to represent _information on resources available on the Web_. To this end, a critical point is represented by the fact that RDF model _metadata_ about such Web resources, which can be defined as data that describe other data. Thanks to metadata, RDF can easily implements some very important functionalities over the Semantic Web, such as: resource representation, querying resource on the Web, discovering resources on the Web, resource indexing, resource integration, interoperability among resources and applications, and so forth. RDF is primarily meant for managing resources on the Web for applications, not for end-users, as to support _querying the Semantic Web_ (e.g., [37]), _Semantic Web interoperability_ (e.g., [38]), _complex applications_ (e.g., [39]), and so forth. Looking into more details, RDF is founded on the main assertion that resources are identified on the Web via suitable _Web identifiers_ like _Uniform Resource Identifiers_ (URI) and they are described in terms of _property-value_ pairs via _statements_ that specify these properties and values. In an RDF statement, the _subject_ identifies the resource (to be described), the _predicate_ models the property of the subject, and the _object_ reports the value of that property. Therefore, each RDF resource on the Web is modeled in terms of the triple (_subject_, _predicate_, _object_). This way, since Web resources are typically connected among them, the Semantic Web represented via RDF easily generates the so-called RDF graphs. In such graphs, RDF models statements describing Web resources via nodes and arcs. In particular, in this graph-ware model, a statement is represented by: (_i_) a node for the subject; (_ii_) a node for the object; (_iii_) an arc for the predicate, which is directed from the node modeling the subject to the node modeling the object. In terms of the popular _relational model_, an RDF statement can be represented like a tuple (_subject_, _predicate_, _object_) and, as a consequence, an RDF graph can be naturally represented via a _relational schema_ storing information on subjects, predicated and objects of the corresponding Web resources described by the RDF statements modeled by that graph (e.g., [40]). Figure 2: Different Web media for the same book (**left**); and their corresponding relational schemas (**right**) [41]. Figure 2, borrowed from [41], shows an example where information on the Web regarding the book _\"The Glass Palace\"_, by [PERSON], occurs in two different sites in the rest of different media, one about the English version of the book, and the other one about the French version of the book. According to this, the two different Web resources are represented by two different relational schemas (see Figure 2). Figure 3, instead, shows the corresponding RDF graphs for the two different Web resources, respectively. As highlighted before, one of the goals of RDF graphs is allowing the easy integration of correlated Web resources. This is, indeed, a typical case of Web resource integration because the two RDF graphs, even if different, describe two different media of the same resource. It is natural, as a consequence, to integrate the two RDF graphs in a unique (RDF) graph, even taking advantage from the \"natural\" topological nature of graphs, as shown in Figure 4. In particular, the integration process is driven by recognizing the same URI that identifies the book resource. Another nice property of the RDF graph data model that makes it particularly suitable to Semantic Web is its \"open\" nature, meaning that any RDF graph can be easily integrated with _external knowledge_ on the Web about resources identified by the URIs it models.