Identify the job by title and let the recipient know how you heard about it. Summarize your qualifications for the job, specifically your work experience, activities that show your leadership skills, and your educational background. Ask for an interview, stating where you can be reached and when you will be available. If your prospective employer is located in another city and you plan to visit the area, mention the dates for your trip. If you are applying for a specific job, include any information pertinent to the position that is not included in your resume.
There is a tendency to think that the often arbitrary rules of letter writing don’t really matter, that as long as your recipient can work out what you’re saying, that’s good enough. This shows a great deal of disrespect for your recipient, though, and for your own ideas. It suggests that a) your reader’s time isn’t valuable enough for you to write clearly and efficiently, and b) that you don’t particularly care about the content of your letter.