In a previous post, What You Need to Know About the Semantic Web, I shared my letter to Harvard Business Review's editor about their recent semantic web article. I realized after I made that post that I should probably have addressed the difference between the Semantic Web (a proper noun) and the semantic web (lower-case). The Semantic Web is a formal extension to the World Wide Web that attempts to add semantic data to the web. There are all sorts of terms and acronyms associated with the Semantic Web such as the Resource Description Framework (RDF) and the Web Ontology Language (OWL).
The semantic web (lower-case now) is a more loosely defined term. Generally it refers to a variety of efforts to make the existing web more semantic. The Semantic Web is certainly one part of the semantic web. However, there many other semantic web efforts other than just the Semantic Web project. These efforts often overlap, intertwine, and compete.
My personal favorite semantic web related project is Microformats. Microformats start with making existing web content semantically structured, instead of structured for presentation. See my previous entry on Plain Old Semantic HTML (POSH) for more information on this concept. Microformats then use HTML markup (i.e. elements, classes, ids) to define data structures based on existing formats. For example, the hCard Microformat "is a simple, open, distributed format for representing people, companies, organizations, and places, using a 1:1 representation of vCard (RFC2426) properties and values in semantic HTML or XHTML." There's at least one browser extension that can extract hCard data from web pages. I'd be surprised if search engines haven't already started using this semantic data to better index content.
Another very interesting Microformats related project is XFN (XHTML Friends Network). In Jonathan Butler's recent blog post on CRM, VRM, and the tip of an iceberg he highlighted Tom Ilube's statement that "the semantic web would cut out the intermediary and restore control of personal information to the individuals who are its true owners.” XFN is a great example of this in action. It allows individuals to define their relationships simply using hyperlinks. Taken to an extreme, a global graph can be made of all of these relationships making the "walled gardens" of existing social networking sites obsolete.