Streaming music service Pandora has never offered an on-demand streaming service as such, but is looking to do so soon, according to the Wall Street Journal. I actually like the current Pandora model; you enter and artist or song and Pandora finds similar music.
For example, if you entered "James Taylor" into Pandora, you would get some items by James Taylor, and some by other artists in a similar style. The offerings stream one after the other, and you have limited ability to pick and choose. Even so, it's an interesting way to perhaps find new music or artists.
The new optional subscription service should be more like other streaming services such as Spotify, where you can pick a specific song at any time.