What is job application cover letter?
The cover letter is attached to the first page of the resume in order to introduce yourself and your interest in the said job.
No resume is completed if no cover letter accompanies it.

Why it is important to write a good covering letter when applying for a job?

Be the hiring manager for a moment, and think about you scanning lots of job applications. Which resumes would you read? Would you read the ones with cover letters or resumes with no cover letters?
Most likely, you read resumes which their cover letters ‘convince’ you to go on and read the resume.
This is why having a well written cover letter is so important.

But first, be sure that your resume is 100% completed before you start writing the covering letter for any job application. Needless to say, that your resume should present how you match the jobs you’re applying for.
For the resume writing task, refer to the index page – Resume Writing Examples| Resume Writing Tips.

Covering Letters for Job Applications – Where to start?

While you are looking for a job, you probably will find some interesting job openings. Write down the following details:
1. Job position.
2. Job description and main job requirements.  
3. Company name, location and some detail about the hiring department etc.  
4. Your main/key competencies for that specific position – why are you a good match?  

Then, make a short research to verify some basic info about the company and the job. You should be ready to apply for the job after having the above four details in hands.

How to Write a Cover Letter for a Job Application that works?

The covering letter may include (some or all) of the following details and actually ‘speaks to’ the recruiter. Here is the stracture/template:
Introduction -
Your details
The job details – Job title and number + where you saw the job opening.

Body -
Who you are and why you want the job -   What type of professional you are. How do you match the job requirement?
Tell about your work status – What are you doing now etc.  
You may include information about – Why you want to change your job and apply for the said job. You can also mention your key skills.
Be yourself – As opposed to the resume, in the covering letter you can add your personal touch. It is the best piece of paper where your personality can be expressed. Therefore, tell why you are interested in this particular job and why you like the company in question.

Ending-
Final statement – A statement that will attract the reader to read the resume.

Remember that, your objective when writing the covering letter is that your resume will be read by the employer, getting him selects YOU for the job interviewing stage.
Further reading – how to write a cover letter for a resume.
The article discusses in details the approach you may take for different types of job application & cover letters.  

Writing a cover letter for job application – Samples

For selecting the proper template when writing your covering letter, refer to:
1. Resume cover letter samples – The article provides sample cover letters for job application. It gives you 3 templates you can customize to write your own cover letters when submitting job applications.

2. Cover letter for job application submitted by emailEmail Cover Letter Examples for Job Applications.

3. Cover letter for career changers who apply for a new jobCareer Change Cover Letter Examples.

4. Some recruiters require/instruct you to include your salary requirements in the cover letter. Obviously, unless it is required by the employer, you shouldn’t provide these details so early. Read more on how to tactfully handle this issue – Cover letter with Salary Expectations.