A resume is an employer's first impression of you and your skills so it is vitally important to have a resume that effectively communicates your skills and experience.
A cover letter should always accompany a resume unless for some reason an employer asks that you not provide one. A cover letter is your opportunity to highlight your skills, experience, and education and how they can translate to the employer’s expectations.
Interviewing is your opportunity to expand on what you have built at the beginning of your application process. Use your interview to connect the job description to your application materials.