XLink - XML Linking Language specifies constructs that may be inserted into XML resources to describe links between objects. XPointer - XML Pointer Language specifies constructs that support addressing into the internal structures of XML documents. In particular, it provides for specific reference to elements, character strings, and other parts of XML documents, whether or not they bear an explicit ID attribute. RDF will provide a uniform and interoperable means to exchange the metadata between programs and across the Web. RDF data model Describes resources named by URIs Model for representing named properties and their values.
Properties represent attributes of resources relationships between resources.