Well, different natures effect pokemon's stats differently, Eg.
A pokemon with a Modest nature with have 10% more Sp atk than usual, and 10% less Atk than usual.
Different natures effect pokemon's stats by 10%, They increase one stat by 10% and decrease one by 10%.
That is my summary, but to see Pokemaster's summary, just click the link.
http://pokemondb.net/mechanics/natures