Common Tag is an open tagging format developed to make content more connected, discoverable and engaging. Unlike free-text tags, Common Tags are references to unique, well-defined concepts, complete with metadata and their own URLs. With Common Tag, site owners can more easily create topic hubs, cross-promote their content, and enrich their pages with free data, images and widgets.
This is great news for developers, many search engines are using this new format to better understand the real meaning behind enriched tagging.