An interrogative pronoun asks a question. They are exactly the same words as a relative pronoun but relative pronouns do not ask a question.
Who is the teacher?
What is for supper?
Note: the following sentence has a relative pronoun, not an interrogative pronun.
I love books which speak well of liberty.
In Greek, the interrogative pronoun is exactly like the indefinite pronoun except the interrogative pronoun always has an accent on the first syllable.
Interrogative Pronoun, τίς, τί, who, what?
See the presentation here and the video here.