“IT’S” WILL ALWAYS MEAN, “IT IS”
Got it?
Not that I mean to yell or be petty, this is purely for emphasis. It is one of the most common, glaring mistakes. “Its” is the possessive of an object (like in the sentence, “That’s its name.”) And for that matter:
“WHO’S” WILL ALWAYS MEAN, “WHO IS”
“Whose” is the possessive pronoun meaning ownership of an item (like in the sentence, “That’s Rex the dog whose owner is my friend Bob.”).
Here is a paper that has more common possessive pronouns that may help you (click for a bigger picture):
