Hi Smita,
Writing a cover letter often seems like a particularly daunting task. However, if you take it one step at a time, you'll soon be an expert at writing letters to send with your resume.
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. A cover letter should complement, not duplicate your resume. Its purpose is to interpret the data-oriented, factual resume and add a personal touch. A cover letter is often your earliest written contact with a potential employer, creating a critical first impression. (Extracted from about.com)
For more on the subject please visit Cover Letters
Regards,
Julie