iCloud & iCloud Drive
http://www.apple.com/icloud/
http://support.apple.com/kb/PH2608
https://www.icloud.com
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?
https://support.apple.com/en-us/HT207428
iCloud Security
https://support.apple.com/en-us/HT202303
E2EE (End-to-end encrypted data)
https://support.apple.com/en-us/HT202303
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
https://support.apple.com/en-us/HT201104
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