Chris has a way with words and presents flawlessly. He gives a great introduction to using Flickr and Delicious. He explains how tagging works and gives an interesting use of tag clouds using US presidential speeches which have been presented as tag clouds. I'd recommend this video to anyone new to the world of tagging.
When I introduce tagging to students I compare tags to the words you might use to look up information in an encyclopedia index. I ask them think of tags as keywords. What keywords would they use if they were searching for information on that idea or topic.
Image used under a Flickr Creative Commons Licence: Uploaded on June 23, 2008 by kenleyneufeld at http://www.flickr.com/photos/kenleyneufeld/2606012255/sizes/s/