What is the difference between WH-interrogatives and relative clauses?

The difference between the interrogative pronouns who, whom, and which and the relative pronouns who, whom, and which is in their use.

An interrogative pronoun introduces a question:

Who was at the door?

To whom should I give the notice?

Which is your apartment?

A relative pronoun introduces a relative clause, a group of Words that relates to the antecedent which precedes it:

The man who was at the door was the super.

The one to whom I gave the notice was the super.

The notice which I gave to the super was about a scheduled power outage.