Exactly as mdross said, there's no specific format. Some applicants are more creative with the format while others use a more generic essay format.
One of the most fun essays I ever read was one where the applicant answered the prompt by first wondering how her favorite authors would have answered it. She did this by writing a paragraph from their points of view. Then she wrote from hers. It was really amusing to read and I enjoyed it much more than the standard essay format.