System databases of SQL Server includes master , model , msdb , distribution.
The model database is a template, used by SQL Server when creating other databases, such as tempdb or user databases. When a database is created, the entire contents of the model database are copied to the new database.
The msdb database is used by SQL Server, SQL Server Enterprise Manager, and SQL Server Agent to store data, including scheduling information, and backup and restore history information.
The distribution database is used by the replication components of SQL Server, such as the Distribution Agent, to store data including transactions, snapshot jobs, synchronization status, and replication history information. A server configured to participate either as a remote distribution server or as a combined Publisher/Distributor has a distribution database.
The model, msdb, and distribution databases are backed up in the same way as user databases and should be backed up regularly if they are changed.
For more detail information , please see the SQL Server documentations.