What is the difference between a main idea question and a detail question?

The basic difference between the two types of questions is:

A main idea question asks for the broad definition of the topic -

For example, What is Harry Potter about? You would give a general answer that tells about the story. (Its About a Boy who finds out that he is a wizard.)

But a detail question would want an answer that contains more information -

For example, Who is Harry Potter? You would give an answer telling us about the character, his personality, where he came from, who his parents were, etc. ( Harry Potter is a young boy that finds out he is a wizard after his parents have died and has been sent to live with his relatives.)

Hope that helps.