The HTML pages on the Web contain human readable text and computer readable formatting – or “markup.” The inventor of the Web, Tim Berners-Lee, also conceived of more broadly meaningful markup – tags like HTML (XML actually) that can be interpreted by a computer to represent a specific piece of data. The term “Semantic Web” refers to using these more advanced tags to create a computer representation of what your page means.
For example, an HTML page can contain an event date formatted in bold text using HTML. This date can be surrounded by additional tags the tell the computer reading it that it is in fact an event with a specific title, day, month, and year. The language and standards of these semantic tags is called RDF – Resource Description Framework.
If Web page authors were to annotate their pages with an agreed set of tags, the Web would become a database of information that computers could use to make inferences – not simply search the text of as they do today. These are huge preconditions – agreeing these tags and using them, but the results would be powerful.
This very fascinating article, which I’ve read returned to and read three times over the last year describes what might happen if Google were to take charge and lead us into the Semantic Web future.
Researchers at the University of Washington have done some interesting real-world experimentation in this area.
Lastly, for a more in depth introduction to the Semantic Web, try this article.
Update:
I found a nice critique of the semantic web here.
Descriptions of the Semantic Web exhibit an inversion of trivial and hard issues because the core goal does as well. The Semantic Web takes for granted that many important aspects of the world can be specified in an unambiguous and universally agreed-on fashion, then spends a great deal of time talking about the ideal XML formats for those descriptions. This puts the stress on the wrong part of the problem — if the world were easy to describe, you could do it in Sanskrit.
The critique points out big weaknesses in the ability of the semantic web to make reasoned judgements. However, I think there is still a lot of capability to be explored in simple additional markup for things like want ads and calendar events.