What is the format and content of the cover letter?
I have covered this is detail here
A cover letter is not standard, but can be based on a template, which must be tailored to be relevant for the particular vacancy for which you are applying. It must be personally addressed, reference the vacancy and the company, and your suitability for the position being advertised.
Very few people send a cover letter, only about 10%, so this alone marks you out as being interested in this particular vacancy, rather than sending your CV off for every vacancy. It tells the recruiter you are professional, and have taken the opportunity to communicate directly.
You can show a little of your personality, and this should at least get you onto the interview list
You can clarify anything that is not covered in your CV, that doesn’t fit into the format. Maybe you want to explain a gap in your CV, or highlight how appropriate one of your previous roles make you for this one.
Elaborate on relevant experience in more detail than a CV allows, express an interest in some aspects of the company, this helps you show you have done your research, and have an appreciation of the role for which you are applying.
Here are a couple of samples you could base your cover letter on.
I have inserted several alternative paragraphs into this example cover letter, select the most relevant for each application, do not use all of them in any application. The first and last paragraphs will probably be constant, and one or more of the others, as relevant. Alter or complete all the items in red as appropriate.