When consulting reference works, we are used to finding information about items by searching for them with higher order categories - thining about groups they are a member of and searching under those groups. An important job for someone organising knowledge is therefore to "tag" things with such category membership.
We may do this tagging for our own benefit - inventing categories that fit our needs. We may want to do this for internet material. For instance, web browsers provide us with tools to "bookmark" pages that we want to revisit and we may organise these bookmarks into our own system of folders
This personal tagging can really come into its own when we chose to share what we do with others. Website services like "deli.co.ous" offer us a way to do this. We can then look at the bookmarks or tags that other people have made that are the same as our own. There is a good chance that we will find they have categorised other things that we would be intereseted in.
This is a distinctive new way to manage our relationship to the information space of the internet. It works not in the normal way: not because it is a good, externally-imposed, standardised classification scheme. Its value depends upon an informal process of categorising but one that exploits social processes of coordination and shared interests. Here are video elaborations of social bookmarking (left) and information tagging (right)...