Master: Master database contains instance metadata and SQL Server system configuration settings but system objects are stored in resource database.
Model: Model database is a template for creating a new database.
Msdb: Msdb database is used by SQL Server Agent.
Resource: Resource database is a read-only database that contains all the system objects that are included with SQL Server.
Tempdb: Tempdb database is a resource that is available to all users is used to hold temporary objects.