Functions of SQL Server default six tables

Master Database
The master database records all system-level information of the SQL server system. It records all logon accounts and system configuration settings, and all other databases, including the location of database files. The master database records the initialization information of SQL server. It always has an available backup of the latest master database.

Tempdb Database
The tempdb database stores all temporary tables and temporary stored procedures. It also meets any other temporary storage requirements, such as storing worksheets generated by SQL Server. The tempdb database is a global resource. All temporary tables and stored procedures of users connected to the system are stored in the database. The tempdb database is re-created when SQL Server is started. Therefore, the database is always clean when the system is started. Temporary tables and stored procedures are automatically removed when the connection is disconnected, and no connections are active when the system is closed, therefore, no content in the tempdb database is saved from one session of SQL Server to another.

By default, when SQL server is running, the tempdb database will automatically grow as needed. However, unlike other databases, each time the database engine is started, it is reset to its initial size. If the size defined for the tempdb database is small, automatically increasing the size of the tempdb database to the size required to support the workload may become part of the system's processing load. To avoid this overhead, you can use alter database to increase the size of the tempdb database.

Model Database
The 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 content in the model database, and the rest is filled with blank pages. Since SQL Server creates a tempdb database every time it starts, the model database must always exist in SQL Server.

MSDB Database
The MSDB database is used by the SQL Server Agent to schedule alarms and jobs and record operators.

Northwind Sample Database
The northwind traders Sample Database contains sales data from a fictitious company called northwind traders, which is engaged in the import and export of specialty foods from all over the world.

Pubs example Database
The pubs sample database uses a book publishing company as a model to demonstrate many options available in the Microsoft SQL Server database. This database and its tables are often used in the example described in the document.