Wikipedia has an extensive article on tags, but the short version is:

A tag is a keyword or descriptive term associated with an item as means of classification. Tags are usually chosen by the author/creator of the item being posted. Typically, an item will have one or more tags associated with it. Someone quickly scanning these tags would be able to determine what the post was about.

On this Web site, blog entries, polls and events can be "tagged."

How can tags help me use this Web site?

As a reader, you'll be able to look at the "tag cloud" on the front page of this Web site and also by clicking on "Tags" in the menu. Here, you'll see all the tags that are being used on the site. If one tag appears larger than another, that means it has been used more often. By clicking on a tag, you'll be able to read every post that has been "tagged" with that term.

For instance, if you were interested in music, you could click on the "Music" tag to look at all the posts that were classified as "Music."

When you are posting something, you'll be able to give readers a quick idea of what your post is about by "tagging" it. For instance, if you are posting an exercise class in the events calendar, you could tag that post as "Health and Fitness" and "Classes." If you were blogging about your opinion on the latest school board decision, you could tag that post as "School board" (and "Taxes" and any other tags that might apply). You are limited to 4 tags per post.