A business letter (or formal letter) is a formal way of communicating between two or more parties. There are many different uses and business letters. Business letters can be informational, persuasive, motivational, or promotional. Business letters should be typed and printed out on standard 8.5" x 11" white paper.
The most important element of writing a good letter is your ability to identify and write to your audience. If you are addressing your letter to the department of human resources, avoid using highly technical terms that only engineers would understand, even if your letter is addressed to an engineering company, chances are that the personnel in human resources does not have an engineering background.