When you activate Backup in iCloud Settings your iPhone or iPad will automatically perform one complete back-up every day - but only when the device is locked and not in use, and is being charged and connected to a wifi network at the same time. That daily back-up includes photos and videos, just like the iCloud Photo Library, but it also includes data from other apps too, such as text messages, health data that is stored in HealthKit apps, and even details of how you've organised all your apps on your Home Screens.
Turning off the iCloud Backup option doesn't affect important apps, such as Mail and Contacts, so your most important data is still protected even if this option is turned off. It's still worth using Backup every now and then though, just in case anything goes wrong with your iPhone or iPad. If you don't want to use Backup via wifi - which could take a while - you can also perform a manual back-up by using a USB cable to connect your iPhone or iPad to iTunes on your Mac. That's a good option too, as it allows you to store the back-up on your Mac's hard disk, rather than taking up the limited space in iCloud.
iCloud and Apps: iCloud Security Code
There's one other important type of data that can be stored on iCloud. The iCloud Keychain stores private personal data, such as passwords for email accounts and web sites, and even credit card numbers that you use on sites such as Amazon or eBay. This is very important information, obviously, so you can keep this information extra safe by creating an iCloud Security Code. If someone gets hold of any of your Macs or iOS devices without your permission they would need to know both your normal iCloud password and the extra iCloud Security Code before they could access your Keychain.
I tend to keep Keychain turned off on my iPhone, so that some important passwords that I have on my office iMac aren't shared with the iPhone when I'm away from the office. But if you want to turn Keychain on then you can simply tap Keychain in the iCloud Settings panel and then click the On/Off button that appears. Underneath that button you'll also see the 'Advanced' option, which allows you to set up the additional iCloud Security Code. There are two options here. You could choose to use your normal iPhone Passcode as the Security Code too. But, of course, if someone manages to get hold of your Passcode then your iPhone is completely defenceless. It's better to create a separate Security Code that has no connection with your normal Passcode. You can either opt for a simple four-digit Security Code or let your device generate a longer, random code number for you.
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