Last week Google took the wraps off Google Music Beta.
In a nutshell the service allows you to now store all you music online so you can listen to all your music no matter where you are. You will have near limitless storage and the ability to auto sync your music and playlists. If you music is stored in multiple locations - not a problem, you will be able to aggreate all your music including iTunes. When you add a new track it will auto save to your Google Music Account. The service goes beyond plain storage by allowing you to "Create your own custom playlists with just a few clicks" or "use Instant Mix to automatically build new playlists of songs from your collection".
What happens when your offline? Well any music you have already listened to will be able to be played offline from that device.