In an interesting post, Tom Coates identifies an apparent shift between two emergent paradigms that have occured in online tagging.
Some people, says Coates, use tagging as a form of folder-style grouping, allowing data entities to be grouped as they would group files in a folder. Tags, used in this way, remove the need for a literal file-folder hierarchy:
It was just that now an object could exist comfortably in a number of folders so you didn't have to enforce an arbitrary heirarchy on your filing...
The second paradigm is that of Flickr, whereby everyone is encouraged to tag everyone's data. As well as this social, collaborative tagging, Flickr seems to highlight a growing tendency towards descriptive tagging (singular, such as 'blog' or adjective such as 'cool') rather than classificational tagging (plural, such as 'blogs' or 'ideas').
Coates evidences this with statistic from his weblog, which shows a trend towards descriptive tagging by users of his site.
Quite a cool and informative post. People have also published some interesting comments and identified some other work that's going on in this area.