Michelle Tullier A cover letter is an introduction, a sales pitch and a proposal for further action all in one. It gives the reader a taste of what's to come - not by simply summarising the resume, but by highlighting the aspects of your background that will be most relevant to the reader.
Cover letters are typically one page documents.
These three components often amount to three or four paragraphs, but there are no hard and fast rules about exactly how you break up the information.