SNMP (Simple Network Management Protocol) is a network protocol that allows one device to query another device for information, change information on a device, or for one device to send a message to another device. It is widely used to monitor hardware and software components of web sites during normal operations or during testing. That is why advanced load testing tools, such as WAPT Pro, usually have ability to connect to any servers and receive performance counters data (such as CPU or RAM usage values).
Older versions of Windows, like XP, had SNMP service installed by default, so you only had to start it or enable automatic start. After that you could connect to that system with any SNMP client and monitor anything you needed.