While the app is running, the app sends a HTTP (edit: HTTPS) request every time the track information changes or the volume changes. When the track information changes it sends the artist, album and song name. When you change the volume it sends the new volume level.
Every request includes standard meta-data such as
* An _anonymous-id_
* Device serial number
* Information about whether wifi or cellular are connected and carrier name
* Device name, model and manufacturer
If there is interest I will write a blog post about potential ways to stop the data collection without removing the app :)