I'm going to 'stand on the shoulders of giants' and let others do the explaining. You can get a flavour for what the semantic web is all about from this non-geek introductory video:

(http://www.youtube.com/watch?v=OGg8A2zfWKg)

You can see an example of how the semantic web can be used with wikipedia here:
http://mqlx.com/~david/parallax/
Their video is also quite enlightening and well worth the 8 minutes.

Having data properly tagged with a well structured taxonomy enables you to use faceted search. Here's an example of a faceted search user interface:
http://orange.sims.berkeley.edu/cgi-bin/flamenco.cgi/nobel/Flamenco

Monaco IQ's sister web site maBoum.com has been semantically enabled from the start. We use microformats on all contact details. This can make your life easier if you use tools like the Operator plugin for Firefox. If you want to try it, please keep in mind it's experimental and if you have any feedback - good or bad - then please let us know. Incidentally, we tried using the events microformat, but the experience wasn't great at the time. We'll certainly revisit it again soon though.

Most activity on the semantic web so far has been quite academic, but some interesting commercial activity has been starting in the last year or two. OpenCalais from Thomson Reuters is a good example.

When writing this page, I checked the link to the OpenCalais web site as I hadn't visited for some time. Unfortunately, it looks like the text has had a 'makeover' from the marketing department and, to put it bluntly, they've messed it up. It now seems to be steeped in marketspeak and is much less clear than it was previously. I wish people wouldn't do that!

If you've now got an appetite for the semantic web, here's some more reading