The most enjoyable and understandable introduction to the Semantic Web I came across for ages was given by Stefano Mazzocchi at the ApacheCON 2007:
A no-nonsense introduction to "semantic web" technologies
The so-called "Semantic Web" is a vision for an evolution of the web where web sites expose data not only for direct human consumption (as it is mostly the case today) but also for specific software agents to consume, aggregate and enrich on behalf of humans. In this presentation, I will show an outline of this vision, together with a simple and concise description of each W3C recommendation (such as RDF, OWL and SPARQL) that are involved and how they are supposed to work together. I will also show the differences and similarities between this and other models for purer-data interchange on the web (such as "web 2.0" and "atom/rss") and will demo existing solution that are based on semantic web technologies.
