When you purchase the Premium Service, the iCloud Backup feature becomes available in the Settings screen of this app.
iCloud Backup is not an automatic sync. It never runs on its own; you back up your data to iCloud manually.
If you use the same Apple Account, you can also share data with another device such as an iPad. Both devices refer to the same iCloud backup data, so if you create or overwrite the backup on your iPhone and then restore it on your iPad, your iPhone's data is transferred to the iPad.
With this feature you do need to tap the backup and restore buttons manually each time, but it lets you share data across multiple devices.