The purpose of a cover letter is to capture the reader's attention. It should convey why you are interested in the company (especially if you are interested in a specific position) and emphasize the three or four characteristics of your candidacy you feel make you an exceptional candidate. As with most correspondences, a lengthy cover letter is not necessarily a better letter. Strive to write in a concise and direct manner. Remember that your goal is to capture the reader's attention and make a good first impression.
Print your cover letter on the same paper that you use to print your résumé. The letter should have one-inch margins and contain the same heading and font as your résumé. Spelling and grammatical errors are completely unacceptable.