This is a circuit for educational purposes. It shows how to make a Clap Toggle Switch using the Arduino board. A basic RC circuit filters the microphone input. The output of this circuit then feeds an Arduino input. The Arduino then drives an LED to indicate that it sensed a clap.
From what I can tell from the circuit and the code, there is a lot to be done before it can be a reliable clap switch. Environmental noise can easily trigger this circuit. But it is definitely a very good start! The complete instructable is found here