What is a leading question?when do we use leading questions?what is a close ended question?when is it best used?

Your question makes me differentiate between3 types of questions even though you asked about2 only

1. Opening question vs Closing/concluding question
2. Open-end question vs closed-end question
3. Leading (suggestive) question

The first category of questions is related to the timing and sequence of these questions within an event, say an interview or an investigation or in terrogation

1. Opening questions are raised at the beginning to establish the subject and prepare for detailed and in-depth questions
2. Closing questions are raised at the end of the event to summarize the main points, secure concurrence with what you concluded, and possibly ask confirm next sequence of  actions

The2nd category of questions is irrelevant of timing and it is more about dictating or enforcing the trype of answer you want.  

1. Open ended questions are less restrictive as in "How can I help you ?"
2. Close-end questions are restrictive with a Yes or No answer   ....  as in  "Can I help you ?

The3rd category of questions is suggestive and implies some answers within their text .... as in "To prove you have no relation with this crime, tell us where were you on8th of June ??