Google Play Music for Android and Google Play Movies & TV for Android have been upgraded and add support for Chromecast. There doesn't seem to be much more in terms of features other than some bug fixes and small user improvements which make context menus easier to access.
You can download the update directly on your device through the Google Play Store or by using the link which follows:
Google Play Music for Android makes it easy to discover, play and share the music you love on Android and the web. With our new All Access service, you can play millions of songs on Google Play, listen to radio with no limits, and enjoy playlists handcrafted by our music experts.
With both All Access and Standard versions of the Google Play Music app, you listen to your music collection anywhere. All your music is stored online, so no need to worry about syncing, storage space or offline playback.