A covering letter is the one that goes with your CV when you are applying for a job. Here is a fairly conventional plan for the layout of the paragraphs.
It is important to remember that cover letters are sales letters, and you are the product. Cover letters serve to introduce you to an employer, and resumes tell about you. Like the resume, the cover letter is an important marketing tool.

Remember, you do not get a second chance to make a first impression. Your cover letter creates your first impression, and you want that impression to be positive.

Purpose of the Cover Letter

It is important to keep these objectives in mind when putting together your cover letter. A cover letter serves to:
•    Create a favorable first impression
•    Introduce yourself
•    Demonstrate your professionalism
•    Illustrate your communication skills

Position requirements often list "good communication skills." Demonstrate those skills through your cover letter. Make sure to use proper punctuation, capitalization and grammar.

Cover Letter Sections

A cover letter has three general sections. The first section should create interest and explain why you are writing. It should state the type of position you are applying for, as well as how you heard about the company. Also in your first paragraph, refer to your enclosed or attached resume.

The second part or paragraph should explain your interest in this type of position. You should relate your qualifications to the job opening, and use action verbs to describe your experience. This should be brief. Be specific in what you have to offer the company.

The third part is the closing paragraph. Here you request action, such as an appointment, an interview, or some other definite action. Make this action easy for the reader to take by listing the best time to call you, the best number to reach you on, and/or your email address to contact you electronically. Close by restating your interest in the company.