In a clear move to drive sales, Google is now allowing users to install Android apps directly from their Google+ streams. When a Google Play app link is pasted into a new post, an 'Install' or 'Buy' button will be attached to the post which when clicked will take a user to Google Play with a prompt to purchase and/or install the app to a device of choice.
This also works with individual music tracks when shared directly from Google Play, with the addition of a 30-second preview of the song directly in the post. As of yet movies and books do not provide direct purchase functionality, however we expect they will at some point.
Source: The Next Web