It is available as a website and also has clients for all the major mobile platforms, which can play streaming music or cache songs for offline playback. There are also clients for the Roku and Sonos systems.
Rdio library has content from the four major record labels, as well as the Merlin Network and the aggregators IODA, INgrooves, The Orchard, IRIS Distribution, and Finetunes. It also offers social networking, allowing users to share playlists and follow others to see what music they listen to.
Rdio was created by Skype founders Niklas Zennström and Janus Friis.