Who's - whose

Whose and who's sound exactly the same. But they are spelled differently, and have rather different meanings: academic teachers hate it if you use the wrong one. Both are derived from the wh- word who.

• Whose is the possessive form. It means 'of whom', and comes in sentences like "Whose is this?" (~ 'to whom does this belong?', or less formally, 'who does this belong to?' (see who - whom), and "This is the friend whose bike I borrowed."
• Who's is the contracted form of 'who is'. (Contracted forms, like isn't, I'm, she's and can't should be avoided in academic writing.)