Who

Who is used only of persons.

Who is there?
Who won the first prize?
Who will be the next Chairman?
Whose is this coat?
Whom did you invite to preside over the meeting?

Which

Which is used for both persons and things.

Which is your bag? (Here which refers to a thing.)
Which is your son? (Here which refers to a person.)
I don’t know which of them will actually get it?

What

What is used only of things.

What did you pay for this?
What is this made of?
What did you do then?
What did you say?
What on earth do you mean?

Difference between which and what
Which and what are often possible with little difference of meaning.
Which is preferred when we have a limited number of choices in mind.

Which color do you like – red, green or blue?

What is used when the speaker is not thinking of a limited number of choices.

What language do they speak in Myanmar?

Difference between ‘What is he?’ and ‘Who is he?’

When you ask somebody what he is, you are referring to the person’s profession.

When you ask somebody who he is, you are referring to the person’s name.

What is he? He is a doctor.
Who is he? He is Mr John Mathews.