What is Spotify? Here is the description from the App Store:
Stream millions of tracks from thousands of albums and artists to your iPhone, iPod Touch and iPad. Take your existing Spotify playlists with you on the go - listen to them during your commute, in the gym or on a flight. With ‘Offline mode’ you can listen even when no connection is available. Wirelessly sync your local files to your phone to merge your catalogue with ours, no cables required! Download and get started directly from your mobile.