Who's and Whose
Who's means "who is." The apostrophe is there because it is a contraction. The apostrophe does not indicate possession.
Example: Who is there? Who's there?
Whose means "belonging to whom." It indicates possession without using an apostrophe.
Example: Whose suitcase is this?
Don't let the apostrophe confuse you. You cannot use "who's" to ask who something belongs to.
Common error: Who's suitcase is this? - Incorrect!