I assume this is a homework or assignment question. what did the lessons leading up to this tell you? I'll offer you some reading on the subject:OneTwo Three There is a lot more information readily available if you ask Google the right question. No matter if this is a question from your head or a school based question A big part of the learning is understanding what is happening. That only comes with time and extensive reading or teaching. With my links you can get the reading part of that input.