Cover letter is a letter of introduction attached to document such as a resume or curriculum vitae. A cover letter typically accompanies each resume you send out. Your cover letter may make the difference between obtaining a job interview and having your resume ignored, so, it makes good sense to devote the necessary time and effort to writing effective cover letters.
Cover letter and resume usually provide all the information which a prospective employer will use to decide whether or not you will reach the next phase in the application process: the interview. While your goal is an interview and, ultimately, a job offer, the more immediate purpose of your cover letter in some cases may simply be to gain an attentive audience for your resume.