The Semantic Web is a “web of data” that enables machines to understand the semantics, or meaning, of information on theWorld Wide Web. It extends the network of hyperlinked human-readable web pages by inserting machine-readable metadata about pages and how they are related to each other, enabling automated agents to access the Web more intelligently and perform tasks on behalf of users.
The Semantic Web, Wikipedia
So, The Semantic Web is supposed to be the next big thing, right? That’s not really news. A lot of websites have organized their content with tags, vocabularies and categories for a long time, but what is interesting, is that we are starting to use some of this semantic markup now, and in some pretty creative ways.
One example is the way some new mobile devices are organizing your contacts and photos. Tagged content like Facebook status updates, tweets, Flickr galleries and so forth is automatically identified as relevant content and is added automatically to your contacts. All of this is naturally kept up to date and in sync via the cloud.
Anyway, all this semantic web stuff can be hard to wrap your head around. This short documentary, in plain english, is a good place to start.