It is a good idea whenever you send a resume to accompany it with a cover letter. The purpose of a cover letter is to introduce yourself, personalize your resume, and to get yourself an interview. It allows you to emphasize your skills and/or abilities that relate to the job for which you are applying.
The person to whom you address the cover letter should be carefully chosen. If possible, try to get the name of the person best qualified to evaluate your skills and most interested in hiring you - the decision maker. If your only choice is the human resource department, make sure to address the cover letter to a specific individual in that department, or address your cover letter to the hiring committee (Dear Hiring Committee) if you know that you are going to be interviewed by several people.
Every cover letter you write will be more effective if you specifically tailor it to the position that you are applying for. Use simple, direct language and keep it to one page. The tone of the letter should be positive. There should be no grammatical or spelling errors. Be sure your letter draws a connection between the needs of the current job opening and the skills you can bring to the job.