Have to say that I think Hatem Mahmoud has done a great job in contextualising and explaining web 3.0/ semantic web in this presentation.
Takes you through web 1.0, through to 2.0 and on to 3.0. Explains why web 3.0 is required and gives some current examples.
Worth spending 15 minutes – for anyone new to semantic web.
This 6 minute video is also an excellent introduction to the semantic web.