A Cover Letter is a self-marketing tool.
It is a letter that should always go along with your résumé. It should not be copied from some generic template but rather, be customized. This letter introduces and highlights major points found in your résumé and communicates your enthusiasm and desire to contribute to the employer's company.
Make sure your cover letter identifies the
position and states why you are applying for it, shows that you made an
effort to research the company, draws the reader's attention to your
qualifications, indicates you have good written communication skills,
and finally asks for an interview.