When applying for a job, both a cover letter and a resume should always be included. A cover letter is generally included along with a resume as a more personal account of why an applicant is suited for a given job. Including unique details such as personal background and interests, a cover letter is often a chance for people to emphasize qualities and experience that may not be apparent from examining their resume.
By addressing your cover letter to the head of the human resources department personally, you are showing that not only have you done your research, but also that you are not simply sending out a generic resume to every company you apply to. If you do not have that person's name, call the company and find out. In the letter, make more specific references to the particular position you are interested in. This will show your potential employer that this letter is directed at their company only. This is your first opportunity to make an impression on the company. Making mistakes at this stage could mean the company might not even look at your resume.
Remember these key details:
Include your address and present contact details
Make it brief
Make it specific to the company you are applying to
Don't forget to take the opportunity to highlight your strengths in light of the job you are applying for. Show as much as possible that you are suited for this particular job.