Windows Management Instrumentation (WMI) is one of the common interfaces used to retrieve information from a Windows system. Such information can include values provided not only by the system itself, but by various software programs running on it. That is why WMI is widely used to monitor the performance and correct operation of the server components.
Many load testing tools, such as WAPT Pro, can monitor performance counters directly from the tested servers with help of WMI. The most useful counters are those representing CPU, RAM and network usage, but there can be other server-specific ones that you can easily retrieve.