There are four main parts of iCloud: Syncing; Backups; Keychain and iCloud Drive. In this article, we're going to focus on Syncing and Backups. But there's also links to information on Keychain, iCloud Drive and Apple's Today at Apple Classes.
What is iCloud?
iCloud is, in plain English, a storage solution provided by Apple that allows you to save information on it and sync it between your devices. In iCloud you can store photos, videos, documents, music, information from apps, backups and more—
If you have an iPhone, iPad or Mac and at some point you downloaded an app from the App Store, then you have an Apple ID (Apple Account) and therefore, an iCloud Account.
What is my phone saving on iCloud?
First, let's see what's going to your iCloud. For this go to Settings > Your Name > iCloud.
At the top you will see your overall iCloud storage and how is used. They all look different. Apple gives you 5gb for free. After that there are 50gb = $1/month; 200gb = $44.99/year ($3.75/month); 2TB ($9.99/month).
The 50gb is a good option to start saving your information on iCloud.
Syncing
All the items that are checked with green, are going to sync between all your devices where you are using the same Apple ID.
So for example, if you have an iPhone and an iPad with the same Apple ID, and you enable the Contacts to sync, you will be able to see the Contacts saved on iCloud, on both devices.
All the items that show in the following image in green, they are set to Sync.
One very important thing to keep in mind is that, when information is syncing in this way, if you delete a contact, for example, on one device, it will also delete it on the others.
Backups
iCloud can also hold backups of your devices. In plain English, a backup is a carbon copy of your device. This, unlike Syncing saves everything on your device including things like the your settings, your wallpaper and the layout of the apps at the moment that the backup was completed.
To enable iCloud Backups go to Settings > Your Name > iCloud > iCloud Backup and turn it on.
The first backup of a device can take up to 8 hours, depending on how big the device is.
Your device will attempt to create a backup every time your device is connected to WiFi, locked and charging.