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!