PBS is the Proxmox Backup Server, basically it does differential backups of your VMs, integrity checks, and allows you to back up to multiple different servers. Proxmox does have a basic backup system that can save the VM images to disk though but I typically recommend the full PBS if you're building a cluster.
One interesting thing you can do is run PBS as a giant VM and back up or migrate the whole thing, just like you can run a small NAS as a VM as well.
One interesting thing you can do is run PBS as a giant VM and back up or migrate the whole thing, just like you can run a small NAS as a VM as well.