A covering letter is a letter of introduction which is sent along with the Resume. The covering letter, in effect is the first communication that one has with a prospective employer.
A covering letter can be an e-mail or a hard copy. The basic difference is in the format rather than the content. In an e-mail, the address and the phone numbers of the sender are below the name of the sender at the end of the letter.
A covering letter is vital in any job application because in a way it helps form the first impression of the candidate.
A covering letter can be either a letter of application or a letter of inquiry. The contents of the letter would wary accordingly.
In either case, it should be brief and well-written without being rambling and ideally, a covering letter should not exceed a page.
At the outset, the covering letter should include a brief introduction of one-self and also specify why the letter is being written. For example, if one is applying for a Customer Service job, it should be mentioned at the beginning of the letter.
One should be specific about the job one is applying for and also how you came across the job opening. In case of a letter of inquiry, you can mention the source, a friend or an acquaintance who sounded you off on an upcoming job opening. For instance, it could be a Training Internship job, which was suggested to you by a mutual acquaintance.
The letter should also briefly mention how you are qualified for the job and how you have the required and desired criteria for applying for it. If one is applying for a Management position, you can highlight how you are qualified to take it on.
A covering letter should highlight the key achievements without going into too many details, as the details are given in the Resume.
It should reflect the communicative skills, attitude, enthusiasm and motivation of the person, so as to help form a positive image of the applicant.
A covering letter must end with thanking the prospective employer for giving the time and opportunity.