A cover letter is a brief introduction to complement a resume or CV, while resumes and CVs provide a summary of skills and work history. Most jobs require a resume, whereas a CV is more in-depth. CV stands for "curriculum vitae" and is used to apply for academic positions.
A cover letter offers the job applicant an opportunity for introduction, as well as some context to the person reading a resume or CV. The cover letter serves to make a positive impression on the employer and is considered just as important as a resume in professional jobs.
The resume is a concise document that showcases an applicant's most relevant skills, experience and education. Typically, it is no longer than a page, but extensive resumes can rate a second page. The resume offers a quick glance at a potential employee's credentials.
The CV, however, is the most in-depth of the three documents. The CV has a focus on academic achievements, such as publishing and research, as well as the information that would be included in a resume. In addition, the CV contains professional affiliations, references and areas of interest. Because of how in-depth a CV is, it should be updated frequently.