A new SQL Server 2005 installation always includes four databases: master, model, tempdb, and msdb. It also contains a fifth, "hidden" database that you will never see using any of the normal SQL commands that list all your databases. This database is referred to as the resource database, but its actual name is mssqlsystemresource.
The master database is composed of system tables that keep track of the server installation as a whole and all other databases that are subsequently created. Although every database has a set of system catalogs that maintain information about objects it contains, the master database has system catalogs that keep information about disk space, file allocations and usage, systemwide configuration ...