Microsoft SQL Server has four system databases

Microsoft SQL Server has four system databases:

The 1.master database master database records all system-level information for the SQL Server system. It records all login accounts and system configuration settings. The master database is a database that records all other databases, including the location of the database files. The master database records the initialization information for SQL Server, and it always has a backup of the most recent master databases available.

The 2.TEMPDB database tempdb database holds all temporary tables and temporary stored procedures. It also satisfies any other temporary storage requirements, such as storing SQL Server-generated worksheets. The tempdb database is a global resource, and all temporary tables and stored procedures for all users connected to the system are stored in the database. The tempdb database is recreated every time SQL Server starts, so the database is always clean when the system starts.       Temporary tables and stored procedures are automatically dropped when the connection is broken, and no connection is active when the system shuts down, so nothing in the tempdb database is saved from one session of SQL Server to another session. By default, when SQL Server is running, the tempdb database automatically grows as needed. However, unlike other databases, it resets to its initial size each time the database engine is started. If the size defined for the tempdb database is small, then each time you restart SQL Server, the size of the tempdb database is automatically increased to the size required to support the workload, which may become part of the system processing load. To avoid this overhead, you can use ALTER DATABASE to increase the size of the tempdb database.

The 3.model Database model database is used as a template for all databases created on the system. When the CREATE DATABASE statement is issued, the first part of the new database is created by copying the contents of the model database, and the remainder is populated with empty pages. Because SQL Server creates the tempdb database each time it starts, the model database must always exist in the SQL Server system.

The 4.MSDB database msdb database is used by the SQL Server Agent to dispatch alerts and jobs, and to record operators.