who

Whom has fallen into disuse in everyday speech, with who taking its place, especially in British English. Do you remember whom you saw? is more usually expressed as Do you remember who you saw?, and whom is omitted when it is associated with a preposition: the man I was talking to rather than the man to whom I was talking. However, in formal contexts, whom is still preferred by careful writers. Note that whom is incorrect in sentences such as The woman who we thought was dead is still alive: who is the subject of was, not the object of thought (We thought that she was dead...).

Whose means "of whom" or "of which" and denotes possession or association: the children whose [not who's] father we met, a car whose [not who's] paintwork had been damaged. (Some people dislike the use of whose to mean "of which," but it is a well-established use.) Who's is a contraction of who is or who has: the friend who's [not whose] coming to dinner next week; Who's [not Whose] got my pen?

See that.