Now we know what it is.
A person in the recording industry who has been briefed on Google's plans says that the service will allow users to share songs "on a limited basis" after purchase.
The beta of Google Music, which launched in May, already lets users upload their collections to a virtual "locker," then stream those songs to any connected device.
Now, users will be able to "send" songs they have purchased to their friends. Those friends will be able to listen to those songs a limited number of times, or for a limited time period, for free.
This person was not briefed on the exact mechanics, but the service will probably work by adding all purchases to the buyer's music locker. Then, the buyer will be able to send an email to their friends with a link to the song in their locker. The friends would then be able to stream the song by clicking the link.
A few years ago this would have been unimaginable. But as shown by the recent launch of Spotify and other services that let users share music for free via Facebook, the music industry has become much more amenable to this kind of sharing because it spurs discovery.
Our source also tells us that Google—like Spotify and many other services—are paying the major labels huge up-front advances to get these kinds of rights. The majors are big enough to demand these advances, while most smaller indie labels are not, and are therefore getting left out of the chain.
This person also said that Google Music will let users "pin" songs to their mobile devices. Presumably, this means users will be able to choose certain songs to cache on their devices, so those songs will be playable even without an Internet connection. But in general, Google Music won't have on-demand caching like the mobile versions of subscription services like Spotify and Rhapsody.
Our source reiterated reports that the Google Music will be a straight per-song download store, with no subscriptions. It is expected to launch by the end of the year.