What is the difference between 'who's' and 'whose'?

The pronoun 'who' is used for the subject of a sentence or a clause.

The pronoun 'whom' is used as the object of a verb or a preposition.
The pronouns 'who' and 'whom' are interrogative pronouns and relative pronouns.

An interrogative pronoun introduces a question. Examples:

Subjective: Who gave you the book?

Objective: To whom will you give the book? (object of the preposition 'to')

A relative pronoun introduces a relative clause (a group of Words with a subject and a verb that gives information about its antecedent). Examples:

Subjective: The one who gave the partywas my neighbor.

Objective: The one from whom I received an invitation was my neighbor. (object of the preposition 'from')