Apple's iTunes business exceeded $4 billion revenue last quarter. Horace Dediu broke down where all the sales are coming from on his site, Asymco. As you can see, it's mostly a mix of content and app sales.
The interesting thing, Dediu notes, is that growth has been steady at 29% each quarter. This is happening despite the competitive environment where Netflix, Spotify, Google, and Amazon are all trying to chip away at Apple's iTunes franchise in one way or another.
Another fun thing about Apple's iTunes business: It's huge. On an annual basis, it's bigger than Yahoo, Facebook, and Netflix combined.