Your cover letter accompanies your CV or application form in an application. A well-written cover letter will highlight why you are applying for this position and why you are the best candidate.
It will outline how you meet the essential and desirable criteria in the person specification and will convey your enthusiasm for the job.
It will also explain how this job will fit in with your career plans, and explain any career gaps or other things on your CV that may need more detail.
Make sure that you save the job description and person specification carefully. If you have downloaded these from a web page, remember that the information will disappear once the closing date for applications passes. You will need this information when you prepare for your interview.
The ideal cover letter is one side in length, anything longer won't be read.
If you are printing, rather than emailing your CV and covering letter, make sure that all printing is done on good quality paper and on one side only. Employers will photocopy your application and then send the copies to the shortlisting panel. Double sided printing risks getting missed out in the photocopying process!
Don't be tempted to put your cover letter and CV on unusual sized, coloured, or patterned paper. Whilst it would be unfair of an employer to disregard it for this reason alone, you want to avoid irritating the shortlisters!
To send a cover letter that has not been carefully tailored to the job for which you're applying implies that you are not particularly interested in the job that you're applying for, and it's insulting to your potential employer.
Stories abound of applicants sending in cover letters addressed to people at other organisations. This is not acceptable and you will need to carefully tailor the letter to each application, meticulously checking that you have all of the information relating to the job and the person to whom you're writing correct.