Used to introduce a clause giving further information about a person or people previously mentioned. The teacher, who was very kind, helped me with my homework.
Used to ask about the identity of a person or people. Who is coming to the party tonight?
Used alone as a question requesting the identity of a person or people, often expressing surprise or seeking clarification. “Someone just knocked at the door.” — “Who?”
Used in indirect questions to refer to the identity of a person or people. She wondered who would show up at the meeting.