If you have been using SQL Server 2005 for a while now, you may wonder what I am talking about when I refer to the SQL Server 2005 Performance Dashboard. No, you are not suffering from overwork and fatigue, causing you to lose your mind and forget what new features have been included with SQL Server 2005. The SQL Server 2005 Performance Dashboard is a new add-on to SQL Server 2005 that became available shortly after the release of Service Pack 2 for SQL Server 2005.
In brief, the SQL Server 2005 Dashboard is a custom report (custom reports are a new feature of Service Pack 2) for Management Studio that gathers data from the many Dynamic Management Views (DMVs) and Dynamic Management Functions (DMFs) available in SQL Server 2005. It is not a replacement for other performance tools, but an adjunct tool to help DBAs better identify and troubleshoot performance problems. In many ways, the SQL Server 2005 Dashboard reports are similar to the many built-in reports already offered by Management Studio.
More there : http://www.sql-server-performance.com/articles/per/bm_performance_dashboard_2005_p1.aspx