ROON APP
A Roon controller is a software remote control for your Roon service. It's not the core, it's not your library. It's simply the way you access what you've got and the way you ask your Roon Core to play music on your devices.
The Roon controller comes in the form of apps. There are Roon control apps for iOS, Android, Windows and Mac OS. They're all created with a single code base to make sure that the experience is uniform.
You browse and play music with Roon apps. All the data is kept synchronized in real-time and Roon claims that there's no lag and no stale data.