Whose is the possessive form of who and which. It can be used to refer to both animate and inanimate antecedents as in I saw my sister, whose favorite season is fall or the tree whose leaves were red. Whose is not to be confused with who’s, a contraction of who is or who has.

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. Who's is used in an entirely different way. It is a contraction of who is or who has as in Who's at the door?