In SQL Server 2008, Activity Monitor has below five sections namely Overview, Processes, Resource Waits, Data File I/O and Recent Expensive Queries.
This shows the graphical display of Processor Time (%), Number of Waiting Tasks, Database I/O (MB/Sec) and the Number of Batch Requests/second.
Processes: – section will show you the list of all the active users which are connected to SQL Server Database Engine. You can even KILL the sessions from here.
Resource Waits - Shows wait state information. It will help DBAs to identify potential bottlenecks with respect to Memory, CPU, Network I/O, Cache etc
Data File I/O - Shows I/O information for database data and log files. You can troubleshoot DISK IO related issues from here.
Recent Expensive Queries – Shows information about the most expensive queries. You can check the execution plan as well by just doing right click.