Real Dictionary

it's
It's is the usual spoken form of 'it is'.

it's in American English
contraction of: it is

it's in British English
contraction of: it is or it has

its
determiner: You use its to indicate that something belongs or relates to a thing, place, or animal that has just been mentioned or whose identity is known. You can use its to indicate that something belongs or relates to a child or baby.

its in American English
of, belonging to, made by, or done by it
that or those belonging to it: the possessive form of it1, used without a following noun
pronoun: the possessive form of it (used as an attributive adjective)

its in British English
determiner: of, belonging to, or associated in some way with it