This helpful contraction of who is (as in Who's there?) or who has (as in Who's been eating the cheese?) is not to be confused with its homophone, the possessive pronoun whose.

When is it appropriate to use who's versus whose? This commonly confused pair can cause English speakers problems because they sound exactly alike, however they are used in different contexts.
Whose, which entered English at the end of the ninth century, is the genitive case of who; it is used to show possession. While some style guides are not in favor of only using whose to show possession with an inanimate antecedent, this is considered perfectly acceptable by many other style guides. This usage is nothing new to English; Shakespeare and Milton both used whose with inanimate antecedents.