The service lets you upload your music library to Google's cloud and stream tunes on your Android devices and your computer.
Playlists you create in the cloud will automatically sync to all your devices. You can even listen to music while you're offline. According to a post on the Google Blog: "[W]e automatically store your most recently played music on your Android device and you can choose to make specific albums or playlists available when you're not connected."
Music Beta by Google also features an "Instant Mix" playlist creator, similar to iTunes's Genius feature, which mixes and matches similar songs and artists that the user might enjoy.
The service will be available by invitation only in the U.S. and will be free during the beta period.