Default system database description of sql server

Master The master database contains system tables that define the behavior of your SQL Server 2005 system. For example, each database that you add is recorded in a system table, and you can access information about those databases by executing a SELECT statement on the sys.databases catalog view. Similarly, information about all system stored procedures is stored in the master database.

model Each time you create a new database, the model database is used as the template for the database you’re creating. It’s possible, if you’re planning to create multiple databases with the same customizations,
to make those customizations once in the model database.

msdb SQL Agent uses the msdb database to store information about tasks.

tempdb The tempdb database is the scratch pad for the SQL Server system. A new tempdb database is created each time you start SQL Server 2005. Similarly, the tempdb database is discarded when you shut SQL Server down, so if you use the tempdb database to store temporary data, be aware that the data is lost when you shut down SQL Server. If you might need the data at a later time, store it in some other database.