who, whom, or whose

The word who is the subjective or nominative case. It acts as a subject of a clause (The runner who won the race) or as a predicate nominative, that is, a pronoun linked to the verb to be or other linking verb (Who’s who).

The word whom is the objective case. It acts as the object of a verb (We invited whom?) or as the object of a preposition (the politician to whom he was indebted).

The word whose is the possessive case. It acts in place of a noun appearing in the possessive (the person whose views we admire).

The correct use of who-whom-whose is discussed thoroughly in the Parts of Speech section of Grammar.com. Click here for the beginning of that discussion.