RDF, or Resource Description Framework, is a scheme for describing data, typically in the format of triplets:
Subject -> predicate -> object
My house (subject) Is painted (predicate) Blue (object)
RDF is used widely on the Internet to provide context to HTML-structured documents.
RDF is the next revolution following HTML: where HTML provided us with linked, formatted documents, RDF gives us a means to enrich documents with context.
There are many RDF vocabularies available, some of which are highly specialized. Let's take a look at some of the more popular ones: