¿Cuál es la diferencia entre su y es?
Its (sin apóstrofo) indica posesión. Contrariamente a las reglas de posesión habituales en inglés, el apóstrofo se omite en its.

It's es una contracción, una forma abreviada de it is o it has.
What is the difference between its and it's?
Its (without an apostrophe) indicates possession. Contrary to common possession rules in English, the apostrophe is dropped in its. 

It's is a contraction, a shortened form of it is or it has.