The cover letter is an excellent opportunity to link the work history and education listed in your resume to the job that you are applying for. This is especially important if your work experience is not directly related to the position. It is your chance to explain to the selection committee what you have to offer and why you are the best candidate for the position. Your cover letter should consist of three or four paragraphs and should not be longer than one page.
The cover letter should be in standard business letter format.
Include your name and current address at the top of the page. Next
list the recipient’s address. Always address your cover letter
to the selection committee.
For example:
Selection Committee
UCSB Chemistry Department
Santa Barbara, CA 93106
The first paragraph should state your interest in the position. Make sure you include the job title and where you heard about it.
The second and possibly third paragraph is your opportunity to explain how your work experience and/or education have prepared you for this position. Try to write your letter from the perspective of the readers. Why should they hire you? How can you contribute to the department? Explain the qualities you acquired from your work history and education. Use examples to demonstrate your abilities. The most successful cover letters make a connection between you and the hiring managers. While you may include similar information in different cover letters, customize each one to the particular job. You can reference the skills and abilities listed in the online job posting. You may also want to do some research on the department and state your interest in a particular aspect. Almost all campus departments have websites that can be accessed from UCSB’s home page.
In the final paragraph, you may want to summarize your relevant education and experience. Conclude your letter with a request for an interview and a phone number where you can be reached. Be sure to say thank you.
For examples, see: 
Sample
Cover Letters.