A product of UserLand Software and DaveWiner. The simplest decription would be "a subscription-based WebLog ('blogging') tool."
Since there is open access to the object database, scripting language, outliner, and web services (XML-RPC and SOAP) calling interfaces, though, it might be defined as "UserlandFrontier with weblogging documentation instead of the Frontier documentation." However, I don't know enough about the product to say this definitively (ahem. Excuse the pun).
Radio Userland is a browser-based tool for maintaining a WebLog as well as browsing the WebLogs and newsfeeds of others. Radio is one of the primary applications driving the adoption and evolution of RichSiteSummary.