A cover letter or job application letter is your introduction to potential employers. A cover letter or job application letter is your sales pitch to the company you are applying to. A cover letter is not a summary of your resume and it doesn't replace a resume. A cover letter complements your resume. The main objective of the job application cover letter & resume is to get the employer interested in calling you for an interview. See certification tests, CV action verbs & the hidden job market.

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Each cover letter or job application letter should be tailored for the position and company you are applying to. Also see sample application letter.
Always send a cover letter or a job application letter with every job application/ resume submission. No exception.
Check job application closing dates and always apply early.
A job application cover letter should be a brief and simple one page letter.
Use CV action verbs to describe yourself, your skills and your strengths.
Each specific job application cover letter should have information relevant to the description of the advertisement of the position. So, show the skills and expertise that you have that match the description of the job you are applying for.
Address each letter to a specific person. If only the person's title is indicated in the job advertisement, phone the company & ask for the person's name. It will show that you went through the extra trouble to find the person's name.
Just as important as tailoring each cover letter for the position, so is tailoring it for the company you are applying to. You can do this by researching the company you are applying to. Keeping this company info will also be handy at interview time.
Don't be fake.
Don't mention negative aspects of yourself or your history.
You can prepare a general cover letter, which must be edited for each specific job.
It's a good idea to keep a photocopy/ record to help you remember what you wrote in the letter.
At this early stage, the employer is only interested in what you can do for their Company, so keep that in mind when preparing your cover letter or job application letter.

Also see sample application letter, highest paying jobs, online distance education & work from home.