Is a workspace for holding temporary objects or intermediate result sets.
Is a read-only database that contains system objects that are included with SQL Server. System objects are physically persisted in the Resource database, but they logically appear in the sys schema of every database.
Is used as the template for all databases created on the instance of SQL Server. Modifications made to the model database, such as database size, collation, recovery model, and other database options, are applied to any databases created afterward.
Is used by SQL Server Agent for scheduling alerts and jobs.