How iCloud keeps information up to date across all your devices
With iCloud, you can store files and data in the cloud so you can access them on all your devices. You see the same information everywhere.
For each device, you can choose which apps use iCloud (like Contacts, Mail, Reminders, or Keynote) and which iCloud features are turned on (like iCloud Photos, iCloud Drive, or iCloud Backup), and you can change those settings at any time. See Sign in to iCloud on all your devices and Apps and features that use iCloud.
Store information in iCloud and access it everywhere
When you turn on iCloud for an app or feature, you store most of its information in the cloud, which frees up space on your device. Because the information is stored in iCloud, you can access it anywhere you’re signed in with your Apple ID. To see the same information on each device, just sign in with the same Apple ID.
For example, if you sign in to your iPhone with your Apple ID and turn on Contacts in iCloud settings, the contacts you create in the Contacts app on your iPhone are stored in the cloud.
If you also sign in to your Mac with your Apple ID and turn on Contacts in iCloud settings, you see those iCloud contacts in the Contacts app on your Mac.
Because your information is stored in the cloud, if you make a change in one place, you see it everywhere. So if you add a new contact or change a contact’s email address on your iPhone, those changes are saved in iCloud. The next time your Mac goes online, you can see the updated contacts in the Contacts app on your Mac.
For some apps and features, you can use iCloud to share and collaborate. The content you share stays up to date for everyone. See Use iCloud to share and collaborate.
If you have a device without iCloud turned on
If you have a device that’s not signed in with your Apple ID or that has iCloud turned off for an app or feature, you don’t see information stored in iCloud on your device.
For example, if you don’t turn on iCloud for Contacts on your iPad, you don’t see your contacts stored in iCloud in the Contacts app on your iPad. You only see the contacts you create on your iPad.
If you don’t want to turn on iCloud for an app or feature on your device, you can access some information stored in iCloud from a web browser. See Sign in and use iCloud.com.
If you turn off iCloud for an app or feature on a device
If you turn off iCloud for an app or feature on a device, information on that device is no longer kept up to date with your other devices. However, you can still see up-to-date information on iCloud.com and on your devices that have iCloud turned on for the app or feature.
Depending on the app or feature, when you turn off iCloud, you may be asked if you want to save a copy of the information on your device. You can always make a copy of the information yourself before turning off iCloud. See the Apple Support article Archive or make copies of the information you store in iCloud.
For example, if you decide to turn off iCloud for Contacts on your iPhone, you can do one of the following:
Make a copy of your iCloud contacts on your iPhone: You see copies of your contacts in the Contacts app, but they won’t be kept up to date with your other devices. If you update a phone number on your iPhone, it only changes on your iPhone. And if you update an email address on another device with iCloud turned on, you see the updated email address on iCloud.com and any other device that has iCloud turned on, but you don’t see the update on your iPhone.
Delete all iCloud contacts from your iPhone: All your iCloud contacts are removed from your iPhone. However, you can still access them on iCloud.com and on any device that has iCloud turned on for the Contacts app.
To learn about what happens when you sign out of a device, see the Apple Support article Sign out of iCloud on your iPhone, iPad, Apple TV, or Mac.