Who and whom are pronouns.
You use who when you are asking about someone's identity. Who can be the subject, object, or complement of a verb. It can also be the object of a preposition.
Be Careful!
When who is the object of a verb or preposition, it is followed by an auxiliary verb, the subject, and then the main verb. When who is the object of a preposition, the preposition must go at the end of the clause. Don't use a preposition in front of who.
Whom is a formal word which is sometimes used instead of 'who'. Whom can only be the object of a verb or preposition.
Be Careful!
When whom is the object of a preposition, the preposition must go in front of whom. Don't use it at the end of a clause. Don't say, for example 'Whom are they elected by?'
Who is often used in reported clauses.
Who and whom are used in both defining and non-defining relative clauses.
In relative clauses, you can use either who or which after a collective noun such as family, committee, or group. After who you usually use a plural verb. After which you use a singular verb.
Be Careful!
When who is the subject of a non-defining clause, don't use another pronoun after it. Don't say, for example, 'He told his mother, who she was very shocked'. Say 'He told his mother, who was very shocked'.
Noun | 1. | WHO - a United Nations agency to coordinate international health activities and to help governments improve health services UN agency, United Nations agency - an agency of the United Nations |