In Microsoft SQL Server, the database that stores the system-level configuration information about the server running SQL Server and about each database on the server. This information includes system tables, transaction logs, system configuration settings, login accounts, and database location and initialization information.
The master database is one of four system databases on every server running SQL Server. The others are as follows:
In SQL Server version 7, all databases, including the four system databases, are separate files in the \MSSQL\Data directory on the server. Each database consists of a database (.mdf) file and a corresponding log (.ldf) file. This differs from SQL Server 6.5, in which the master and model databases and the first 2 MB of the tempdb database exist as a single file called the master device.
Be sure to always have a recent backup copy of the master database for recovery from system failure on a server running SQL Server. Don’t bother backing up the tempdb database, but for improved system performance you can prevent tempdb from automatically growing to meet system needs by increasing its default size to its maximum anticipated size.