Microsoft Tag is a mobile application that enables users to link almost anything in the real world to more information or to an interactive experience on the mobile device. Microsoft Tag client software installed on the mobile device allows the owner to use the device's camera to scan or take a picture of ("snap") a Tag barcode, then accesses the information associated with the Tag on the mobile device.

http://blogs.msdn.com/tag/archive/2009/07/23/microsoft-tag-api-announcement.aspx