This is quite a tricky pair and I always have to think carefully before I can plump for the right one. If you make a mistake, it’s not one that the grammar check will always pick up.

Whose is the possessive form of who. It means ‘belonging to whom or which’.  Whose usually sits before a noun.

Examples

The teacher kept the class behind until she found out whose mobile phone was ringing.

Whose book is this?

My son knows the girl whose handbag was stolen.

Who’s is short for (a contraction of) either who is or who has. It has no other uses.

Examples

Who’s coming out with me tonight? (who is)

Who’s forgotten to do the recycling? (who has)

I met the speaker who’s delivering tomorrow’s talk. (who is)