When applying by mail or email, a cover letter is always sent with a resume.  It is used to introduce yourself and your qualifications to an employer.  A cover letter answers two important questions:

1.  Why do I want this particular position with this particular organization?   Do your research.  Learn about the organization so you can say something positive about them and explain why you want to work for them as opposed to their competitors.  When researching an organization, you might focus on their mission statement, introduction of new products or technologies, expansion into new markets, gross revenue or environmental policies. 

2.  Why should they hire me? 
Focus on two or three of the highlights from your educational, work, volunteer or personal experiences and how they will benefit the employer.

• Never exceed one page
• Minimize "I" statements as much as possible
• Single-spaced sentences, double-spaced between paragraphs
• Type the word "Enclosure" two spaces below your printed name