A cover letter is sent with your resume and provides additional information on your qualifications.
If the letter is being sent as a hard copy, then it should be in the proper business format, otherwise, if the letter is being emailed, it should have an appropriate subject line and proper grammar.
I am writing in response to your ad for a Widget Specialist in the Daily Newspaper. As a trained widget professional, I have many years of experience working in the field of widget development. Please find my resume enclosed, which includes a detailed history of my work in the widget industry.
I feel that my experience with widget development and design could be an asset to Widget World. In the past, I have worked with several diverse teams to design new and helpful widgets for today's discerning widget user. Some of my previous projects include:
Previous jobs have given me the ability to work with many types of people and to complete tasks within a given deadline. I feel that these skills, along with my experience with widgets, would be an asset to the Widget Specialist position. Thank you for your consideration. I look forward to speaking with you regarding this position.