The cover letter (US) or covering letter (UK) is a letter that you send along with your resume/CV. The purpose of the letter is to acquaint the prospective recruiter with your unique talents and skills and to try to arrange an interview. Address the letter to the person who has the power to hire you. (You may need to look on the company website or make a telephone call to find out that person's name.) Each letter should be unique (and original) to accompany a resume; you cannot use a form letter. The more personal the letter the better.
While cover letters are perhaps less important today than in the past (due to electronic communication), students may well be asked by a prospective company or organisation to provide a letter of motivation, which is very similar.
See some examples
The layout of the letter is exactly the same as a formal business letter with your present address, city, country, and date in the top right hand corner. The person to whom you are writing, their title, the name of the company and the company's address are put on the left. (This is called the inside address.)
Begin the letter with Dear Mr./Ms./Mrs. (for example Dear Mrs. Jones) followed by a comma (UK) or colon (:) (US). (See the major differences between English and American English here) If you do not know the person to whom you are writing, begin with Dear Sir/Madam.
The body of the letter should highlight some of the skills and experiences that you described in your resume that closely fit the specific job or position you are interested in. There are typically three paragraphs which can aligned on the left-hand margin (as the text below):
State who you are and the purpose of your letter. For example: "In response to your want ad in the newspaper... " or "I am a first year student in a three-year program leading to an M.S. in Management at TELECOM School of Management...". If you have been recommended by someone known to the reader, state this straightaway. "I am writing to you at the suggestion of Mr. Robert Davies..."
Describe your interest, refer to and even restate two or three experiences or skills things that show you match profile requirements.
Mention your desire to further discuss employment possibilities in person, on the phone, by email or on the Internet. It is useful to indicate your availability and your willingness to meet. You can also volunteer to provide further information if necessary. A typical ending is "I look forward to hearing from you soon." Examples
Finish the letter with "Sincerely yours" aligned on the left, followed by your signature and your typed name.