Your covering letter is your first point of written contact with your potential employer and if you lose their interest with the letter they may not even look at your resume. It needs to be professional, clear and short, no more than three-quarters of an A4 page.
The cover letter should tell an employer why you think you’re skills are going to match that position and what you are going to bring to the organisation. It should highlight, summarise, and compliment the other parts of your application. It also allows you to add a personal touch. Briefly show your knowledge of the specific organisation and demonstrate your understanding of the position being offered.