The second key component of your file is the cover letter. Whether you send it by mail or by e-mail, your cover letter should be attached to your Curriculum Vitae and should follow certain rules.

A cover letter should showcase your CV.  It represents a link between your work record (the past) and the job you are seeking (the future). It must demonstrate what interests you about the job for which you are applying. You must show the recruiter that you can be counted on to perform the tasks associated with the job you are seeking. If you succeed, there's a good chance that the recruiter will want to meet you and you will get an interview!

A cover letter is addressed to a single individual with a view to landing a job in a specific company.  This is true even when you are responding to an ad published by a placement agency.