It adds a notification to the Forum for the user in the blue box, and notifies the Forum member by email that they have been tagged in a post (at least that is how it works for me using Google Chrome on Windows 7).
Not that you might ever want to do this @Rob (as I imagine you don't like it, or the 'like' function for that matter!), but to tag someone in this way type the '@' character followed by the first few characters of a Forum member's name, then select them from the list