A letter of application is a sales letter in which you are both salesperson and product, for the purpose of an application is to attract an employer's attention and persuade him or her to grant you an interview. To do this, the letter presents what you can offer the employer, rather than what you want from the job.
Like a résumé, the letter of application is a sample of your work and an opportunity to demonstrate your skills and personality. If it is written with flair and understanding and prepared with professional care, it is likely to be very effective. While the résumé must be factual, objective, and brief, the letter is your chance to interpret and expand. It should state explicitly how your background relates to the specific job, and it should emphasise your strongest and most relevant characteristics. The letter should demonstrate that you know both yourself and the company.
The letter of application must communicate your ambition and enthusiasm. Yet it must be modest. It should be neither aggressive nor compliant: neither pat yourself on the back nor ask for sympathy. It should never express dissatisfaction with the present or former job or employer. And you should avoid discussing your reasons for leaving your last job.
Finally, it is best that you do not broach the subject on salary. Indeed, even if a job advertisement requires that you mention your salary requirements, it is advisable simply to call them "negotiable". However, when you go on an interview, you should be prepared to mention a salary range. For this reason, you should investigate both your field and, if possible, the particular company. You don't want to ask for less than you deserve or more than is reasonable.