A backup is a second copy of something you cannot recreate. Photos, documents and messages qualify. Installed apps and the operating system usually do not, because they can be reinstalled.
Cloud sync services are convenient but they are not backups on their own: delete a file on one device and the deletion syncs everywhere. Real backup keeps history, so you can go back to a version from before the mistake.
The common rule is three copies of anything important, on two different kinds of storage, with one kept somewhere else. For most people that means the device, a cloud service, and an occasional copy to an external drive.