Fluffy's answer is correct about the possessive usages, so I won't repeat those. However, it misses an important point. Possession isn't the only use for 's; it can also be a contraction for is.For example: it's, how's, he's, she's, that's, etc.In standard English, s' is never a contraction *. The double meaning of 's leads to one of the most common mistakes in written English: confusing its ....