Tagging is the application of a mnemonic or category, by a user, to a piece of content.
Tagging is provided for in many internet-facing systems, from social networks (Facebook, Twitter, LinkedIn, Pinterest) to CMS systems, as well more formal information retrieval services, such as those found in libraries. Tagging is not as rigid as the categorization found in information-architected systems, but more free-form, allowing users to make up a category right then and there.
Tags may be coincident with keywords. But I think where keywords are determined by looking over an entire system, tags are determined by the individual users, even while remaining enumerable within those systems.
Tagging was something my friends with TMFT did in the 90′s. It was an expression and application of self, and I think not much has changed besides the medium. For tagging to occur, it must remain an engaging and engrossing activity. Developers have the burden there. And as tagging goes on, those services prepared to adequately capture, report, and act on that data will have a market edge. (Twitter, I know you’re selling it!)