A covering letter gives you the opportunity to show how your skills and abilities fit the vacancy. It can be used to highlight particular parts of your CV that are your unique selling points or to draw attention to additional information that does not fit easily into a CV. It can also be useful tool for explaining any personal circumstances or anomalies in your application.
In short, it is the biggest chance you have to sell yourself.
What to include
Explain why you are writing, making sure it entices the employer to read on. If you're replying to an advert, state what you are applying for, say where and when you saw the advert and if there is a reference number, quote it. If applying speculatively, say what has prompted you to apply.
Experience and personal skills showcase Briefly explain your current job and, if applicable, qualifications (professional/academic). Don't give too much away or they may not want to go on and read your CV. If you are replying to an advert, make sure the skills you specified are reflected in your CV.
Draw attention to the relevant aspects of your experience; say what you learned and how it would be useful for the job or to the employing organisation. Sell yourself on the basis of your personal qualities, making sure that these match the requirements of the job/role. Take the opportunity, if necessary, to explain any anomalies in your background, such as any time gaps. Perhaps explain how any hurdles you’ve encountered have helped you develop in a positive way.
The perfect team Explain why you wish to work for this company and what you know about them. Demonstrate enthusiasm and evidence of research into such aspects as their successes, involvements, values or clients. Remember to research the firm fully by visiting its website or premises, using every bit of information you find to help improve your chances.
A positive sign-off Lay down an action plan; say you would like the opportunity to meet them for an interview and that you will await their response, or that you'll call in a few days (you don’t have to leave the ball in their court but be careful that you don’t pester). Indicate your availability for interview.