Who (pronoun)

1. What person or persons.
2. Used to ask for identification of a person.
3. Used to introduce a clause giving further information about a previously mentioned person.
4. Used to refer to the person addressed.
5. Used to refer to a person previously mentioned or assumed to be common knowledge.

Origin:

Middle english, from old english hwā; akin to old high german hwē who, latin qui who.

Examples:

1. Who is at the door?
2. Who wrote this letter?
3. The man who came to see you.
4. Who do you think you are?
5. Who is coming to the party?