What is the difference between relative and interrogative pronouns?

There are just five interrogative pronouns. Each one is used to ask a very specific question or indirect question. Some, such as “who” and “whom,” refer only to people.

Relative pronouns: These pronouns are used to answer clauses such as who, which, whose, that, where, and when. 

Example: Anna's brother, who loves pickles, made fried pickles for his dinner party. 
 Who loves pickles? Anna's brother. That is the relative pronoun. 

Interrogative pronouns: These pronouns answer a question, but do not answer a clause. These sentences are always in a question form.
Example: What time is the festival tonight?

Hope this helps!