The cover letter is every bit as important as your resume. In fact, many employers will not even look at your resume if the cover letter is poorly written, disorganized or difficult to read and understand. To under score its importance, I saw somewhere on the internet that over 80 per cent of recruiters surveyed by the Society for Human Resources Management said cover letters are a "necessity" and would not even read a resume without cover letter. And of course, if your resume isn't even read, the chance of getting an interview is virtually zero.
A lot of job seekers do make the mistake; sending resume without accompanying cover letter. Sometimes ago, I was discussing with a younger friend of mine, who has been sending resumes without cover letter. I guess a lot of people have this wrong opinion because, usually the ad might say, send your resume to … Anytime you see something like this, make sure cover letter is sent as well.
Doing a cover letter poorly is bad as well. A whopping 76 per cent of recruiters surveyed said they would not consider applicants who had errors in their cover letters. This shows you are not detailed oriented and you have poor communication skill. Therefore, it is important to give the same careful consideration and attention to the preparation of cover letters as you gave to your resume.
General Guidelines and Suggestions for Preparing Cover Letters
Show what you can do for the reader, not what you want the reader to do for you; keep the focus on your qualifications as they relate to the position you are seeking.
Write clearly and simply; avoid jargon and overly complex or lengthy sentences; say what you have to say directly and succinctly.
Business letters are formal rather than informal communications
Keep it brief; a one-page letter of 3 - 4 paragraphs is sufficient; your cover letter should never be more than one page long. It is important to feature only the strongest points in your cover letter, as the letter should be no longer than one page. Almost one quarter of recruiters surveyed by the Society for Human Resources Management said they would not consider applicants who submitted cover letters that exceeded a page.
Mention one or two specific accomplishments or strengths that demonstrate your expertise or proficiency and make the reader want to know more, then refer the reader to your resume for more information.
Always keep the reader in mind; make your letters easy to read and keep to the point; demonstrate that you understand and can use sound business writing principles in your communications.
Produce high quality, error-free copy; follow the guidelines used in preparing your resume and be sure to proofread carefully; use the same font and pitch used in your resume, and make your letter look as much like your resume as possible.

Be honest; always be able to back up what you say with evidence and specific examples from your experience.
Be positive in tone, choice of words and expectations; convey your self confidence, enthusiasm and professionalism.
Here is a template cover letter document that you can use as guide to writing your own killer cover letter.
The Header
Here's the basic format for the header of a cover letter:
To the right, give your address and the date. Immediately below that, specify the recipient's address and a brief salutation.
The Body
Begin the body of the letter by expressing your interest in the position, the title of the position (and any identifying code listed in the job ad) and where you saw the position advertised.
After this brief intro, begin a new paragraph and spell out in clear, simple and brief sentences how your experience parallels that of the job description. As you're doing this, be sure to refer to your resume.
The Closing
Closing the body of your letter is pretty standard fare.
Specify how you can easily be contacted, the most convenient being your email address, and thank the recruiter for his time. Below is a sample cover letter

Nyerere Avenue,
Maitama Sule, FCT
Abuja
Monday, November 25, 2013.

The Personnel Manager:
Application for the Post of Business Development Executive
I hereby apply for the position of Business Development Executive which I saw on the internet. I am extremely interested in the opportunity to work with Aiico Insurance Plc.
Smart, Intelligent, goal-oriented, self-motivated and proactive. These are the attributes that I have and which I believe will assist me in contributing to the growth of the company. I also work well under pressure, require little or no supervision. I can inspire others to succeed. These attributes coupled with my educational background will stand me in good stead in contributing my quota to the advancement of the company, and letting it achieve its vision and mission. I’ve also included my CV for your perusal.
I am looking forward to being part of your great team. To contact me, my e-mail address is: gambosoji@yahoo.com. Enclosed is my resume that gives the full range of my skills and abilities. Hoping to hear from you soon.
Thank you very much for your time and consideration.

Someone is this post: https://www.nairaland.com/1314470/please-review-cover-letter#16083863 is asking people to help him or her criteque a cover letter and somebody wrote: Kindly state your career iobjectives here. Career Objectives in a cover letter? That is wrong. You don't include objective in a cover letter. That belongs in the resume.
For more information about writing a good resume, see the following threads:

https://www.nairaland.com/1448096/how-write-first-grade-resume
https://www.nairaland.com/710075/how-write-attractive-resume-need

To even make things simpler, you can log on to this website:
www.getemployedquickly.com?source=CoverLetterDoingitRight&website=Nairaland
and download a program for free that can help you generate excellent resumes and cover letter

Thanks Misterman! So many people need this.

This cover letter, should is be sent as an accompanying attachment in pdf/word format, or should it be typed directly in the mail? Please enlighten a novice.

@chayooh; You are welcomed. Hope I have been able to assist. Please note that it is for limited time only. Soon, charges will apply

@ Nuges11,
Really, there is no rules pertaining to that. However, I prefer it to be included as an attachment, because of formatting issues. The recommended thing is to include combine both the cover letter and resume together as an attachment, and simply introduce briefly in the e-mail.
Follow the link to go to our website, download the app I talked about, install it and use it. Simply use it to enter your details, make sure you entered every details before submitting. Quickly use it, because it is free for now. Wish you luck in your job hunting efforts.

Thanks man