If you make an effort, others will help you. cv::fitEllipse() does not detect anything. That much is clear from the documentation. You are asking others to reproduce here everything that you can read from the documentation. I can't imagine anyone will want to do that. The docs are there, read them. Save this space for targeted questions that clearly demonstrate prior effort. Claims of future intentions are worthless. What have you tried thus far?
Please ask an informed and specific question after first giving things a try by yourself.
Actually i need to know the difference so that i implement the one who is suitable for me