If you've ever used a CD with MP3s on it in your BMW, then it is about the same, but the Dension uses the 6 CDs of a CDC as different menus for categorizing and other features. CD5 is the main root MP3 directory from which you can navigate by artist, album, A-Z, filename, etc. to the song you want. It will use the steering wheel controls or iDrive knob to select and play. It's pretty seamless and you'd swear that it was a factory MP3 setup except for the "CD5" at the top of the screen.
I have no idea how it integrates into non-iDrive systems.