SQL server2008-Object Explorer-database

Database: System Database (master, model, msdb, tempdb), database snapshot, custom database composed of three parts.

The Master:master database is the most important system database in SQL Server and records the information at the SQL Server system level.
Includes: All login accounts and their passwords, system configuration information, all database information, the primary file address of all user databases, etc., including many system stored procedures to perform certain system operations.
The master database is the most important database in the entire system, and if you lose the master database, it can be very difficult to recover all the user databases. Therefore, it is very important to save the most recent backup of the master database.

tempdb database
The 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 dropped, 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.

By default, when SQL Server runs, 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.

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 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.

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

SQL server2008-Object Explorer-database