Difference between rospy.spin and rospy.sleep

edit

I didn't understand the part "It is used when your node has to do some kind of regular processing in addition to responding to topic events, timer events, etc." ;( Can you please explain in codes what this would mean? If I have two functions in a node. One does image processing and the other one controls the robot. When I use rospy.sleep(20) after each function, It just waits 20 sec to run the loop again. rospy.spin() I used like this-

if __name__ == '__main__':  
    try:
        talker()
        main(sys.argv)
        rospy.spin()
    except rospy.ROSInterruptException:
        pass

Here main() has a call to another function in it's code. So my question is does rospy.spin() run this try: block of code again and again?

@Shivam Please open a new question, as comment section is NOT meant to ask a new question.