Apple iCloud provides Apple users with storage space to back up the content of their iDevices. It currently offers a 5 GB free storage and up to 50 GB paid storage.
An iCloud backup runs daily when your iDevice is connected over the Wi-fi, to the Internet, plugged into a power source, locked or when you force Create a backup directly from the iDevice: Settings > iCloud > Storage & Backup > Back Up Now.
To manage your iCloud storage you can easily do that from your iDevice: Storage & Backup > Manage storage.
iCloud backups are stored in Microsoft (Azure) and Amazon (AWS) clouds in encrypted form and Apple holds the encryption keys to access data from your iCloud backup. An iCloud backup always contains the last 3 stored backups or snapshots (incremental backups for faster downloading).