Tagging: a quick and easy way to classify or categorize things like blogs, photos, slides, etc.
I use them in my blogs and in my photo sharing platforms as a way to help others get to what I've posted. Is it a good way to classify? Well, that depends....in the library, we use subject classification in the hierarchy form, which is the way we teach children to locate items, so tagging would not necessarily be a good thing for getting books in the right places on the shelves or for helping them find the books they need. Our catalogs are based on very specific headings and spelling even counts. There are many school librarians, though, that sort their collection into various subject areas or genres to make it easier for children to quickly find what they are looking for....seems a little like tagging, but in a more formal sense as only one person is assigning labels to collections. I would be very apprehensive to allow patrons to start creating their own tags for items in the catalog since everyone doesn't associate the same words with the same things.
HOWEVER....I think teaching kids to "tag" their own work, whether in blogs or electronic portfolios can be a good thing. I think it reinforces "main idea" or "gist" because it allows them to think of the "big picture" in order to come up with tags that are appropriate to their work.