What is a Tag?

Google defines the noun 'tag' as 1) A label attached to someone or something for identification or other information.

On websites, a tag is a non-hierarchical keyword or term assigned to a piece of information. Tags help describe an item and allow it to be found again.

William Web Design has an excellent explanation of what tags are used for and why on their website.  The only difference here is that the tasgs appear at the top of the post.