The Developers API on Google Backup gives some further insight on how the backup stuff is supposed to work (and several questions here show how it really works - that is, sometimes it does, sometimes only partly, and sometimes not at all). Relevant questions here covering those issues include:
a list of the apps installed from Google Play.some application data (if the apps support it explicitly).Switching on a new device for the first time asks whether one wants to store one's data with Google, which then would always sync 'some stuff' to the Google servers, basically It is nothing new that one can use multiple Android devices with a single Google account.
