These terms have the same pronunciation but very different meanings.
A. it's = its ( possessive form of it ): is a possessive adjective for things and animals, similar to his and her.Example :
1. The boy broke his leg.
2. The girl broke her leg.
3. The dog broke its leg.

B. it's ( short for it is, it has ): is a contraction, short for "it is" or "it has".Example :
1. It's not raining. ( It is not raining.)
2. It's late. ( It is late.)
3. It's been raining for hours. ( It has been raining for hours.)
4. Do you think it's arrived?. (Do you think it has arrived?)