The differences between iCloud backup, iCloud Drive, and iCloud Photo Library are confusing, to say the least. It’s always best to consult Apple support for the definitive steps to using any of Apple’s backup and cloud-based storage. Below is the Apple support webpage that I found most helpful. It details how to backup via iCloud and iTunes. Both of these methods create files that are meant to restore your iPhone. One is stored in the cloud (iCloud backup) and the other on your computer (iTunes backup).
When enabled on your iPhone, iCloud backup automatically backs it up each night when WiFi is enabled and while it is being charged. For iTunes backup, your phone has to be physically attached to your computer with the Lightning adapter that you use to charge your iPhone with.
Only relying on iCloud backup requires a certain amount of faith in Apple’s servers which I don’t have. That’s why I like to make a secondary backup to iTunes on an occasional basis that is safely stored on my computer—which is then backed up again to an external hard drive or another cloud-based backup service. The biggest advantage with iCloud backup is that any data you add to your phone is backed up every 24 hours, whereas the iTunes backup is only as current as the last time you got around to doing it.
There is another difference between the two backups: If you have Photo Library turned on, the iCloud backup does not back these images up because they are already stored on iCloud.com in Photos and pushed to all your Apple devices. iTunes backs up everything on your phone, including your photos. Here is yet another reason to make periodic backups with iTunes: if you accidentally delete an image on your phone, it automatically gets deleted on iCloud.com. If you have an iTunes backup that you made after that photo was taken but before this deletion, it will still be there to be retrieved.
Keep in mind that neither of these backup methods are meant to be have their contents revealed to you, though there is third party software like Wondershare that can read the iTunes backup file and allow you to access the contents. Each of these backup methods is really intended to restore your iPhone after a reset, or to transfer data to a new phone.
I will tackle iCloud Drive and iCloud Photo Library in a future post. Learning this stuff is a journey, not a destination.