Welcome to the Metadata Millennium: A Complete Overview of What Metadata Can Do for Publishers
Another more complex but more powerful method is Resource Description Framework (RDF), usually expressed in XML as RDFa ("RDF in attributes"). This is a core technology of the Semantic Web, and it is already widely used in the library world. It's also at the heart of many advanced search and semantic technologies. Its simple subject-predicate-object structure provides a way to describe the relationship between one thing and another, which ultimately creates a "network" of relationships. Not only does this enable extremely powerful semantic associations, it also enables inference: when you know "Liza Daly [subject] works for [predicate] Safari [object]" and "Andrew Savikas works for Safari," you can infer "Liza Daly and Andrew Savikas are colleagues" without explicitly connecting those dots. Semantic metadata is not something publishers commonly use today, but it is very powerful and likely to become increasingly important.