Here's a quick explanation (on YouTube, so you won't see anything in this space if you're at school):
This video from the Jackson County Public Library (USA) shows a basic concept of tagging - it's on YouTube, and cannot be embedded in this blog.
This video explains tagging in a different way:
There are many ways of classifying things. I might look at this photo I took of a river and think of the words river, frozen, and cold. You might think ice, grey, winter.
Tagging is very subjective. The words you choose to use as tags depend entirely what you see in a picture, or (if it's a web page) what you think a web page is about. That is why "social bookmarking" or "social tagging" is so interesting - someone else may see other things in the photo or web page, and by adding their tags, the possibility of finding the item with keyword search increases.
At the same time, the tagged description may be diluted, because of inaccurate or far-fetched tags.
My tag cloud on del.icio.us - an online social bookmarking site. If I were to go to the site and click on one of these tags, the list of all the web pages I have bookmarked and tagged with that word would open. On del.icio.us, the tag cloud is only visible to the account holder, though the saved webpages are visible to anyone. This is my page at del.icio.us
This is a picture of my tag cloud at flickr.com - a photo sharing site. If you were to go to the website, and click on "autumn", for example, all the pictures I have labled "autumn" would open. By looking at the cloud you can infer what pictures I like to put on the site - and tag. I have hundreds of pictures onine without tags.
Tag clouds can be a way to look at your own wrting.
See a related post at August 28.