Cover letter is letter for introduction or summary of the document attached. Cover letter is first point of interaction. Most of the time cover letter is written to accompany resume or business documents.
Cover letter should be short and simple and giving a summary of letter attached with it. Letter should consist of 2 or 3 paragraph.
Cover letter should be different for every job you are applying. It should be modified according to the profile you are applying for.
In bold type, highlight your skills that match the job description.
Fix all spelling errors, typos and grammar problems.
A well written letter make interesting for the reader to read the details of the document attached.
A well written, highly targeted cover letter greatly increases your chances of getting what you want. It could be a job interview, a business deal, etc