I have been trying to design a pir sensor circuit, when searched all over internet, this is the circuit that mostly seen. Can anyone explain the working of the circuit for me?

 

I am reasonably certain that I explained the operation of exactly this circuit a few weeks back. It does not report sensing IR, it triggers on changes in the IR level above a certain rate and above some amplitude.
What is totally missing from the circuit shown is the actual PIR sensor device.
Very briefly, the PIR sensor is similar to a photo-resistive sensor in the long-wave IR spectrum, and so as a moving target varies the radiation, the resistance varies, causing a variation in the voltage input to the high gain stage U1A. This amplified signal is coupled thruC6 to amplifier stage U1b.
What is not shown is the comparator circuit (U1c) used to deliver the pulse to the triggered timer, (U1d).