What is a cover letter? It's a letter of introduction that highlights your key accomplishments and fit for a job opening. A cover letter adds focus to your resume. Few employers seriously consider a resume that is not accompanied by a cover letter; thus, a cover letter needs to be part of your job-search strategy. Each cover letter must be tailored to each job, each employer.
Why is a cover letter so important? A resume is of limited value to an employer if he or she doesn't know what kind of work you want to do. A cover letter tells the employer the type of position you're seeking -- and exactly how you are qualified for that position.
Your cover letter can explain things that your resume can't. If you have large gaps in your employment history, reentering the job market or changing the focus of your career, or relocating and conducting a long-distance job-search, a cover letter can explain these circumstances in a positive way.