Your application letter should let the employer know what position you are applying for, why the employer should select you for an interview, and how you will follow-up.
When writing an application letter you should include:
First Paragraph: Why you are writing - mention the job you are applying for and where you found the listing.
Middle Paragraph(s): What you have to offer the employer - mention why your skills and experience are a good fit for the job.
Last Paragraph: Say thank you to the hiring manager for considering you and note how you will follow up.