Classification approach that relies on users adding freeform keywords to content. Used on popular sites such as Flickr, Del.icio.us, and 43Things. Often displayed using a tag cloud that scales the font size of a tag with its popularity.

Metadata for the masses. Allows users to add any term without complying with a controlled vocabulary. Facilitates pivoting — discovery of similar content with the same tag, or related tags applied to the same content. Complements other classification approaches.