Tagging is a method of organization - nothing more, nothing less. It's just a way for you to organize your pictures (Flickr), bookmarks (del.icio.us) and blog posts (blogger). It's similar to organization with folders, but instead of putting a picture in a folder, you are attaching a tag (or label) to the picture. This means that instead of having a picture that may cover several different subjects in one folder, you can "tag" that picture with several different tags and find it using any one of them.
Tagging is also social. When you tag a picture in Flickr, you are giving others a way to find that picture. You can do a search at Flickr for all pictures tagged "library" and find all kinds of images of libraries, inside of libraries, around libraries or of personal book/movie/dvd libraries. Whatever anyone decided fit the label of "library" will show up in a search. This also means that once you tag an image with a particular tag/label you can find other images that use that same tag.
Why tag items?
- To make them easy to find again - organization!
- To make them easy for others to find - sharing!