Every morning I manually backup my iPads and iPhone to my iMac. When I connect them I’m given the option to backup or sync. I always select backup but I’d like to know the difference. I haven’t found an answer on Apple’s site.
(I also automatically back them all up to iCloud but I still like to have a local backup.)
Thanks,
Betty
Sync means get the Mac and iOS device in sync. Changes to files and databases as well as new files get copied so both Mac and iOS device have the same latest version. This can go both ways depending on what was changed last and on which device. Your iOS device will sync whatever material you have selected to be synced in the iOS device window in Finder.
Backup means all your personal settings and data are backed up to the Mac so the Mac can be used to set up a new iOS device to mimic your old, or to return an iOS device to the state it had been in when that backup was made. Backups do not contain some personal data if you choose not to encrypt them. But it’s important to also be aware that a backup is not a complete copy as you’d think of a clone. This is by design. The backup does not contain iOS and iOS system files. Also, any media that you download and sync such as iTunes music tracks or TV shows and movies (from formerly iTunes) will be not included in the backup. If you sync your photos and podcasts with your Mac, those will not be included either. The backup just includes a copy of your databases so that when the backup is used to set up an iOS device, the device knows what media it will have to fetch and download from Apple’s various stores. And after that, a sync will then also fetch all the other synced material that resides on your Mac and isn’t in your backup. This is all by design and keeps the backup lean — you can easily tell this by looking at the size of the backup file. It’s much smaller than the total of your iOS device’s occupied space minus iOS system.
So very simple bottom line: you select what content gets synced. Syncable content is not included in the backup. And if you restore a device from backup, the sync after the actual “restore” is what makes the restore complete.
There’s a couple good Apple articles on some of this stuff. I’ll have to Google DDG them first.
Here are a few good ones.
What gets backed up if you choose to encrypt backups.
What does “syncing” actually mean and involve.
What is included in an iOS backup to Mac.
How to back up and restore your iOS device to/from a Mac
Another way about thinking sync vs. backup is this way:
A sync goes both ways. It makes things useful on both devices and ensures the latest version of your stuff is available on either device.
A backup only goes from the iOS device to the Mac. But the Mac cannot do anything useful with that data. It’s only stored there so at another time you can do a restore which then takes data from the Mac one-way to the iOS device. This is all about iOS data only. Unlike synced data, the Mac is merely a storage vessel for backup data and cannot do anything (at least within default macOS) with it.
You cannot do a complete restore without a sync (because backups are not complete clones). You can, however, do sync without ever doing a backup (not a good idea though, you want to back up at least every once in a while).
Simon’s answers are, as usual, correct and comprehensive. Let me emphasize a few points with some concrete examples.
Backup:
An iPhone or iPad backup will let you restore your device if you accidently erase it. Also, if you lose your device and get a new device, the backup will let you copy to the new device everything that was backed up, essentially making the new device a copy of the old device. A backup is nothing more than an insurance policy.
There are some nuances based on things like whether you back up to the cloud or to a Mac and whether your backups are encrypted, (like whether or not passwords are backed up), but those are explained in detail in the articles that Simon linked.
Sync:
Syncing is focused on keeping day-to-day things like contacts and photos the same on your iPhone and your Mac. For example, if you manually added me to your iPhone’s contacts and manually added Simon to your Mac’s contacts, you’d have different contacts on your devices, especially you don’t sync your contacts to iCloud.
Manually syncing would make sure have both contacts on both devices. If you sync to iCloud and both devices are set up to sync to iCloud, they’d eventually get in sync automatically, but manually syncing forces the issue. You have a lot of control over what gets synced, in part because you might not want everything on a mobile device, or maybe you have an old iPhone that doesn’t have a lot of storage space.
Summing Up:
As Simon said, a backup makes sure you have a copy of the important stuff from your iPhone/iPad. Syncing can update data on your Mac, your iPhone, and your iPad to make sure that the same data is available for you to use on all devices.
Thanks Simon. This really helps.
Thank you, @Simon, for the narrative and the links. I had wondered about the difference but never got around to asking. (Thank you, @bettyfellows, for asking.) And thank you, @josehill, for the summary.
Am I correct that this information would generalize to using iMazing rather than the Finder?
Not entirely. iMazing will let you perform a complete backup - including your apps and other data that would normally be restored by a sync operation after a restore. And it can be used to restore everything from your backup, including apps that it backed up, which may no longer be available for download.
At least that’s what people tell me. I haven’t tried it.
I use iMazing to make a backup and let it store multiple versions of an app–just in case there is a problem. One app switched to a subscription model. I used iMazing to restore an older version. It still works–albeit with fewer features.
That’s the pro side of iMazing backup; the con side is each version saved uses space on your Mac so you do need to be aware of that.
That’s what external drives are for. 
Thank you Simon for the detailed answers and the resources but I believe this is a tricky subject with many subtleties.
For example I’m not sure that ‘syncing’ an iPhone/iPad via the Finder (previously iTunes) will transfer the photos both way: I believe that it will copy the photos from you Mac to the iPad/iPhone but you still need to use Apple Photos app (or Apple Image Capture app) to import the photos taken on the iPhone/iPad to the Mac.
Also, if iCloud syncing of Photos, Contacts, Calendars, etc, is enabled on the iPhone/iPad, then ‘syncing’ via cable of their data will be disabled.
Even if iMazing backups can contain more data that the Finder backups (like the actual apps which are not stored anymore by the Finder), both the Finder and iMazing backups will not contain the photos and music that were ‘synced’ from the Mac to the iPhone/iPad. But at least iMazing allows you to import them from the iPhone/iPad to the Mac, and it also allows you to access on the Mac most of the backed up data like photos taken on the iPhone/iPad, iMessage & Whatsapp conversations, etc.
And there still are some messaging apps like Telegram, Signal & Threema (maybe others?) whose data cannot be backed up neither by the Finder nor iMazing and which require a direct transfer from iPhone to iPhone which is impossible to do if the iPhone is lost or damaged.
Yes indeed, that’s a leftover from the old days. New photos still in camera roll have to be manually imported with Photos. Once they have been, however, any changes made on the iPhone (metadata, albums, etc.) will sync back to the Mac. Just like changes to album structure made on Mac will sync over to the iPhone.
Of course for those using iCloud photo sync (or in the past Photo Stream) that goes away, as it does when you use iCloud to sync your calendars and contacts.
This makes sense - you don’t want two different sync protocols interfering with each other.
Some would argue that this is a feature, not a problem. I suppose that you could argue back that this should be a choice for the user, but that would be for the app developers to solve.