An interrogative pronoun is used both to represent a noun and to ask a question.

For example

Who art thou, Lord?
What shall this man do?

The interrogatives are:

•  who, used to inquire for persons.
•  which, for persons and things.
•  what, usually for things.

For example

Who gave thee that authority?
Which house does he live in?
What have I to do with thee?

    
When a definite object is inquired for, what and which are interrogative adjectives used to limit the name of the object inquired for.

For example

What books do you want?
Which road shall we take?

The difference between who, which, and what as interogatives appears in the following:

• Who asks for the name.
• Which for the individual.
• What for the occupation.

For example

Who did that work? Mr. Jones.
Which Mr. Jones? John Jones.
What is he? A printer.

    
Besides pronouns, the following interrogative adjectives are used in asking questions.

• why
• where
• when
• how