Four System databases in SQL Server: Master, model, MSDB, and tempdb.

(1) The master database is the most important database in the SQL server system. It records all system information of the SQL server system. The system information includes all logon information, system settings, SQL Server initialization information, and other system and user database information. Therefore, if the master database is unavailable, SQL Server cannot be started. In SQL Server 2005, system objects are not stored in the master database, but in the resource database.
(2) The model database is used as a template for all databases created on the SQL server instance. Because tempdb is created every time you start SQL Server, the model database must always exist in SQL Server. When the create database Statement is issued, the first part of the database is created by copying the content of the model database, and then the remaining part of the new database is filled with blank pages. If you modify the model database, all created databases will inherit these modifications. For example, you can set permissions or database options or add objects, such as tables, functions, or stored procedures.
(3) MSDB is a proxy service database that provides storage space for alarm, task scheduling, and operator operation recording.
(4) tempdb is a temporary database that provides storage space for all temporary tables, stored procedures, and other temporary operations. The tempdb database is used by all databases of the entire system. No matter which database the user uses, all the temporary tables and stored procedures they create are stored in tempdb. When SQL Server is started, the tempdb database is re-established. When a user is disconnected from SQL Server, the temporary table and stored procedure are automatically deleted.