The internet right now, as Tim Berners-Lee points out in Scientific American, is a web of documents; documents that are designed to be read, primarily, by humans.
The vision behind the Semantic Web is a web of information, designed to be processed by machines. The vision is being implemented: important parts of the key enabling technologies are already in place.
RDF or the resource description framework is one such key technology. RDF is the language for expressing information in the semantic web. Every statement in RDF is a simple triple, which you can think of as subject/verb/object and a set of statements is just a set of triples.
Three example triples might be: Armstrong/visited/moon, Armstrong/isa/human and moon/isa/astronomical body. The power of RDF lies partly in the fact that a set of triples is also a graph and graphs are perfect for machines to traverse and, increasingly, reason over. After all, when you surf the web, you’re just traversing the graph of hyperlinks. And that’s the second powerful feature of RDF.
The individual parts, such as Armstrong and moon, are not just strings of letters but web-addressable Uniform Resource Identifiers (URIs). When I publish my little graph about Armstrong it becomes part of a vast world-wide graph: the Semantic Web. So, machines hunting for information about Armstrong can reach my graph and every other graph about Armstrong. This approach allows the web to become a huge distributed knowledge base.