A cover letter is based on a set up of your information and work experience and salutation and closing and your signature and employer contact information the cover letter is for jobs I guess mainly used for that. The difference between a resume and a cover letter would be that a resume is built up by your  experiences in  jobs u have had any achievements and the basic some background information about yourself and this is used for like  jobs and  high school. The similarities are somewhat would be the format it is used because it  for the cover letter u might want to put is as simple as possible just like your resume you just want to be a little brief on what you are putting .I really see both formats the same because they just make it simple to put things that want to mentioned and real brief. So the cover letter is like an introduction to what you are going to show on your resume.