Performance monitoring and optimization is an inseparable part of a DBA's activities. To optimize a poorly performing system/query or to troubleshoot the performance issues you need to know the root cause. Prior to SQL Server 2005, we had several system tables to monitor the state of the system. Though these system tables are still available in SQL Server 2005 and in later versions but SQL Server 2005 introduced whole new set of Dynamic Management Views (DMV) and Dynamic Management Functions (DMF) to monitor the health of a SQL Server instance, diagnose and tune the performance issues. For details, click here.