The cover letter is used in addition to your CV when applying for a job. A brief synopsis of laying out this type of letter follows:
Your opening or first paragraph should introduce you, the writer, and what job you are applying for. It is also correct to state how you learned of the job opening.
Your second paragraph should state why you want the job. Mention why you want to be interviewed and how you are qualified. List your qualifications, experience, and what personal qualities you possess that would make you their top applicant. Remember, this is the reader's first impression of you. This will either get you an interview or it will not.
Your third paragraph should let them know you have included a recent CV. If there is anything else you think might be relevant to getting your foot in the door, write it here.
Your closing paragraph should list the times you would be available to interview, give a recap of your interest, and a quick thank you. Then, close your letter.