Google Music is free service and just like other Google's products it's web-based services, which you can access online using PC or mobile devices.
And this cloud-based service has already have giant Music labels supported such as Universal, Sony Music Entertainment, EMI and totally 13 million tracks is provided.However this service is currently only available in the United States. It is because Google has not reached the completed negotiations with those music labels in order to allow this service to sell songs elsewhere -- outside US.
And don't worry as users, you will get the automatic back up for iTunes and other music formats like WMA, AAC, MP3, FLAC and OGG. Great isn't it?
You can read more in the official Google Blog.