The master database contains all of the system level information for SQL Server – all of the logins, linked servers, endpoints, and other system-wide configuration settings.
The model database is used as a template whenever a new user database is created. You can change most database properties, create users, stored procedures, tables, views, etc.
msdb is used by the SQL Server Agent, database mail, Service Broker, and other services. If you aren’t actively working with things like jobs, alerts, log shipping, etc.
Tempdb is the workhorse of the system databases. It is the workspace that SQL Server uses to store the intermediate results of query processing and sorting.
SQL Server Tips · · · ·
there is one more database "resource". you can read about that here.