whose and who’s

Whose is a possessive word meaning ‘of whom/which’, used in questions and relative clauses. Who’s is the contraction of who is or who has. Compare:

• Whose is that coat? (not Who’s is that coat?)
  It was a decision whose importance was not realised at the time. (not … who’s importance …)
• Do you know anybody who’s going to Poland in the next few days? (not … anybody whose going …)
  I’ve got a cousin who’s never been to London. (not … whose never been …)

There is a similar confusion between its and it’s: (see here).