There are three main ways of backups in OpenStack: 1) Instance snapshot, 2) Volume snapshot, 3) Volume backup.
Instance snapshots are the attached persistent volumes that live on the Virtual Machine and also the flat files that make up the virtual machine itself. So when you take a snapshot of the VM it’s the exact point in time of the OS, all of it’s files etc. Snapshot of a VM is a great practice before doing a major upgrade on software and or the Operating System.
With Volume snapshots the idea is that you can continually take snapshots that are of the exact point in time of the data within a volume that would reside on the same storage that the volume lives on.
The idea behind taking a backup with cinder is that you can backup a volume to a different platter or storage node that is not the same node that the original volume resides. So in best practice for production you would probably even want to setup your backup storage nodes in different data centers etc.