The intent of a cover letter is to specify the post or the position for which the job application is attached/sent along with the person who is to receive it and a list of the applicants qualifications.
Writing letters is the most effective communication tool as it helps to streamline our thoughts and will ensure that you use the right vocabulary to express your thoughts. Writing will ensure that you are not just thinking out aloud. Most quote that you can judge a book by its cover and this generally holds good for a cover letter too. Your cover letter will make the first impact on the reviewer as it the first document of your application he will peruse.
The role of your cover letter is to make a clear statement of the post you are applying for and that you have the right qualifications to make the relevant post.
It is essential that the first paragraph of your cover letter you indicate the job for which this resume is intended. Ensure that you name the addressee and not make a sweeping -to whom so ever it may concern etc. Preferably enter the name of the Human Resource Officer in-charge.
Next illustrate very concisely how you will be an assest to the company through naming your core competencies. Always ensure that position for which you make out the application fits your qualifications. The detailed competencies should essentially be enclosed in the resume that follows and the cover letter should trigger that the reviewer flips over to your resume.
Additional comments on employer as well as the company in a manner that will indicate that you are familiar with the profile of the company will also impress upon the reviewer that you have an interest in the company.
Commending the company or the firm on how it has grown from being a small start up to reaching the pinnacle within its industry as in a telecommunications giant or a software company will further indicate on why you want to be a part of the company. Besides a brief reference to the people behind the company or their unmatched leadership qualities that you admire and want to imbibe by working with them, will also explain the area of your focus.