Question

how is carbon different from graphite though their formula is same?

Answer 1

Carbon form many different allotropes like diamond, graphite, buckminsterfullerene, etc. 
Different allotropes of carbon have different structures and thus different properties, like Diamond is the hardest known natural material whereas Graphite is very soft material. But they are chemically identical as they are composed of carbon (C). 

Answer 2

as they have different structure
And graphite is a allytrope of the Carbon.