Although there are similarities between a cover letter and a résumé, there is a major difference between them as well. Both are meant to show why you are qualified for a job, but a résumé is more of raw data whereas a cover letter is more specific and more tailored to the job in question.
A résumé is a general statement of what you have done in your career. It lists such things as your education and your work experience. You are generally expected to put all of your work experience in your resume, regardless of whether it is important to the job for which you are applying. Résumés also typically include references. This is information about you that is relatively generic. It tells what your qualifications are, but it does so in a general way.
By contrast, a cover letter is specifically tailored to the job you are applying for. Like a résumé, it tells why you are qualified for a job. Unlike a résumé, it does so very specifically. It takes the information on the résumé, highlights the important points, and relates it to the job you are applying for. In the cover letter, you also explain why you are interested in this specific job with this specific company.
Thus, the difference between these two is that a résumé is more general and a cover letter is very specific to a given job.