Cover letters are typically one page in length with 3–4 short and concise paragraphs:
Your first paragraph should indicate the position title (and number, if there is one) for which you are applying and where you learned about the position (Career Services, internet sources, personal contact). You should express interest in the organization and their product or service and how it matches your career direction.
Your second paragraph should talk about why you are a good fit for the position. You need to talk about how your skills and experience match the requirements of the position. This is when the job description becomes important; you want to use the exact terminology that the employer is using in the job description (Ex. If the company says they want someone with strong communication skills, you should write about your experience in communicating with other individuals from specific current or previous experience).
You can also include personal characteristics that make you a great employee or give you an advantage (Ex. multi-tasking, team player, strong problem solver).
Recent grads should explain how your academic career makes qualifies you for the position. And, experienced professionals should highlight your work experience.
The last paragraph is your closing. It is recommended that you include a statement similar to the following "I look forward to speaking with you about the position and how I can contribute to (insert company name)". You should also repeat your contact information and thank them for their time.
See basic cover letter skeleton.
See samples of a cover letter
Don't forget to format your letter in standard business letter format. According to the Gregg Reference Manual by William Sabin, standard features of all letters include the following:
Return Address (original letterhead heading used on resume)
Inside Address of employer
Salutation (employer name)