If your CV is a store that showcases everything that you have to offer, then your Cover Letter is the store window containing key information designed to entice and encourage the passer-by to call in!
Your Cover Letter should be an insight into what you can OFFER an employer, not what you want from them.
It should detail why you are contacting them, an insight into why you are suitable for the position you are applying to and why you want to work for the specific organisation.
Always consider your audience - would you be interested in reading your content?
Therefore, be succinct, ensure your writing is sharp and make the content jump from the page!