Tagging, Folksonomies & Technorati

Tagging has many different applications.Delicious is a social bookmarking service that allows you to tag, save, manage and share Web pages all in one place. With emphasis on the power of the community, Delicious greatly improves how people discover, remember and share on the Internet.With Delicious you can access any bookmarked sites on the Internet from anywhere, it doesn't have to be the same computer. You can easily share your bookmarks with friends who also use Delicious.
Folksonomies are another form of tagging.They are used to tag photos on sites such as flickr. They appear to used more broadly and could cause some confusion when people are searching for pacific content.

Technorati tags are used to keep track of blogs. A tag is a keyword or short phrase that writers assign to articles to describe or identify the content: the subject matter, the people involved, the type of article, themes addressed. This helps people searching for a particular type of content to find articles using those tags.

Read more: http://technorati.com/tag/#ixzz16dGoaRsR

They are all forms of providing subject headings for the object at hand.