iSpy Server uses your webcams and microphones (and other sources) and creates a server on your local network that the main iSpy application can then connect to and retrieve video and live audio from.
iSpyServer is basically a much slimmed down version of iSpy that doesn't have any motion detection or alerting code built in, and therefore has a much lower processor impact than the main application. (it should be able to easily run in the background whilst people work on the same PC).