When SQL Server is installed the setup program creates the MASTER database device, then creates the master database and places it on that device. The master database controls user databases and the operation of SQL Server as a whole. It keeps track of user accounts, remote user accounts, remote servers that this server can interact with, ongoing processes, configurable environment variables, system error messages, the databases on SQL Server, the storage space allocated to each database, the tapes and disks available on the system, and the active locks.
It is possible to add user objects to the master database, but it is not recommended. Any objects created in the master database should be used for the administration of the system as a whole. Set permissions in master so that most users cannot create objects there. You can discourage users from creating objects in master by changing each user's default database (the database to which a user is connected after logging in). However, the system administrator's default database should remain master.
Important You should back up the master database each time you change it ¾ each time you create, alter, or drop any device or database; each time you create, alter, or drop a database object from master; and each time you execute a stored procedure that changes master.