Please refer to the Netbackup NDMP guide, it explains the concept quite well.
Some key concepts:
1. You cannot install backup application agents on a NAS.
2. You cannot install a backup application's media server on a NAS, therefore the NAS cannot be instructed to write its data directly to tape or backup-target disk locations.
3. When you mount the NAS's volumes/partitions via CIFS/SMB or NFS on a host where a Nbu client is installed, and then you tell Netbackup to backup this mounted volume, the catalog information of the backup job will assiciate the data to the client, and not to the NAS. In fact, in this case the NAS is completely invisible to Netbackup. Netbackup doesn't even know a NAS exists; all it knows is that it could backup files from the client, it would not know that the data actually travels from a NAS via CIFS/SMB/NFS to the client. This would create problems and confusion for some people. If you mount the NAS volume to another Nbu client, you could still backup the NAS volume by changing client in the backup policy. But as you continue to do this, you will create complications for restores. Say you want to restore some files that existed on the NAS volume on a specific date, which Nbu client was it mounted to on that date? You will need to know this to be able to find the files for restore.
4. You cannot do snapshot backups with CIFS/SMB mounts. (I think you could with NFS mounts). Snapshots backups are important for some applications/environments.
Now with NDMP (referencing the above points):
1. With NDMP, you don't need to install a Nbu client on the NAS. You couldn't anyway.
2. With NDMP, the NAS can effectively become a Nbu Media Server without you having to install anything (you couldn't anyway), and it could write directly to tape via direct scsi connections, skipping the LAN and the Nbu Media Servers. This is alot quicker (in most cases) than backing it up as a mounted volume from some client host. the NDMP NAS could even share the tape drives with other Nbu Media Servers via SSO.
3. With NDMP, the NAS has its own identity with Netbackup and this problem would be avoided. When you restore, you specify the NDMP/NAS name.
4. With NDMP, you can do Snapshot backups. Block level snapshot backups even. With CIFS/SMB/NFS you can only do file-level backups.
There are probably many more (and also some NDMP shortcomings), but I hope that helped point you to some good directions.
