'Who's' and 'whose' are commonly confused words in English grammar. Many people mistakenly use 'who's' when they actually mean 'whose'. This confusion arises due to their similar pronunciation and spelling.
'Who's' is a contraction of 'who is' or 'who has'. It is used to combine the pronoun 'who' with the verb 'is' or 'has'. For example, 'Who's going to the party?' means 'Who is going to the party?' or 'Who has gone to the party?'
'Whose', on the other hand, is a possessive pronoun. It is used to show ownership or possession. For example, 'Whose book is this?' means 'To whom does this book belong?'
Using 'who's' instead of 'whose' is incorrect because it changes the meaning of the sentence. 'Who's book is this?' would mean 'Who is book is this?' which does not make sense.