Apple Resources - iCloud

iCloud & iCloud Drive


  iCloud mostly serves to back up your device and sync limited
  kinds of data to other Apple devices, while iCloud Drive
  more focuses on sharing among devices and seamless
  interaction and co-working among devices, which means that
  data in one device can be shared among other devices without
  copying, sending or transferring. For example, you create a
  new Pages file on your MacBook Air, put a few lines on it,
  and then throw it into iCloud Drive. Then you may continue
  working on it wherever you go as long as you have access to
  it via your iPhone or iPad.

What does iCloud back up?
iCloud Security
E2EE (End-to-end encrypted data)
  End-to-end encryption provides the highest level of data
  security. On each of your devices, the data that you store
  in iCloud and that's associated with your Apple ID is
  protected with a key derived from information unique to that
  device, combined with your device passcode which only you
  know. No one else, not even Apple, can access end-to-end
  encrypted information.
Access and manage files in your iCloud Drive