Tags are a free form method of categorization, currently being used in the WordPress Support Forums.
The goal of using these tags in the WordPress Support Forum is to create a "heat map", an index of topics covered within the Forum. The larger the size of the tag, the more discussions about the topic. This provides two options for the user seeking support. They can search through the forum for help or click on the word that best categorizes their issue, and find a list of topics dealing with that issue.
For example, if you are having trouble with comments on your page, click the tag comments in the tag section and a list of discussions about comments in WordPress will be shown. You can scan down the list and possibly find the answer you need.
What if your question is a little more complicated and covers several topics. For example, say that your support question is about a changing the background image on your blog's header. Some tags you could choose may be background, header, header image, and graphics.
When creating a new support thread, there is a space provided for tags. Enter the tags you want, separated by spaces, e.g. background header graphics, in the tag input box at the bottom of your post message.
This will enter in one word tags to your post. If you would like to add more tags, or tags which include more than one word such as a short phrase like header image, add these in the box next to the title at the top of the post. You can add tags after the thread has been started, even if you were not the one who started the thread. Adding useful, descriptive tags will help connect related threads.
Tags are all the rage. Here are some other sites that use the "tags" concept: