Have you looked at the search engines yet? If so... have they not managed to answer this for you? If not... what specifically have you got difficulty with?
I mean to say... if you have researched this in a search engine then you will have come across some explainations - so what is it about those explainations that you don't understand?
Post some more structured questions and you will receive a more willing reply... but phrased as you have, it looks like you have done no work on the subject and merely want the answer on a plate.
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