You might ask why you need a cover letter, but this is the first impression that you can make with the employer and interviewer. If you can make this letter good, your resume and interview will be taken much more seriously. In this letter you should be answering "Why should I hire you?" and "Why did you send me this resume?" for the interviewer.
The cover letter should contain: a showcase of your skills that you have already laid out in your resume, cover what you are offering to the company and the job, factors that the employer might identify as key interest, education and skills, and other contributions that you can provide that would help the company benefit from your employment.
Don't stand out for the wrong reasons in your cover letter, and be remembered for the good things that you have done. Also check your spelling and grammar over many times as this is something professional, and can be the difference between getting the job or unemployment.
The first thing you want on your letter is a formal opening of a letter, addressed to the manager. An address to the manager or interviewer should be first, along with where you are coming from and what you are applying for.
The first paragraph should be some form of an expansion of who you are to the interviewer, something that wouldn't fit into your resume and maybe something that you would like to expand on for the interviewer.
The second paragraph should be about your value to the company could be, such as what you have to offer and why you are more valuable than the next person that they have lined up for an interview.
The final paragraph is a conclusion, but also add in any quick reminders that you have for the interviewer and put a formal thank you for the interviewer's time.
Remember that the cover letter is the first thing that you hand in when you hand in an application package, then your resume and finally an application if the company has supplied one.
