Master

This database is the "brains" of your installation. It contains a great deal of the metadata that describes your instance. Be sure to retain an up-to-date backup of the master database. Back up the master any time you have made changes to your SQL Server installation, including when you have created or modified other databases. Note that you can only do full database backups on the master. You cannot back up its component files, perform differentials, or backup up its transaction log. Recovery of the master database requires special considerations.

msdb

The Microsoft SQL Agent uses the msdb for schedules, alerts, and for recording the backup history. All types of backups can be performed on it, providing that it has the full or bulk-load recovery option set.

Model

The model database serves as a template for new databases when the Create Database statement is executed. All types of backups can be performed on it, providing that it has the full or bulk-load recovery option set.

tempdb

The tempdb is for the temporary databases that applications use. It cannot be backed up and does not appear in the NetBackup for SQL Server backup browser.