HTML can only say that the span of text "ID586172" is something that should be positioned near "Acme Gizmo" and "$199", etc. There is no way to say "this is a catalog" or even to establish that "Acme Gizmo" is a kind of title or that "$199" is a price. There is also no way to express that these pieces of information are bound together in describing a discrete item, distinct from other items perhaps listed on the page.
The semantic web comprises the standards and tools of XML, XML Schema, RDF, RDF Schema and OWL that are organized in the Semantic Web Stack. The OWL Web Ontology Language Overview describes the function and relationship of each of these components of the semantic web:
RDF can be defined in three simple rules:
You've seen facts already. Each line below was a fact:
:john :a :Person .
:john :hasMother :susan .
Current ongoing standardizations include: