No resume should be sent out without being accompanied by a cover letter. Cover letters basically have three paragraphs. The first paragraph states the job you are applying (may include a job number as well) and how you heard about the job. The second paragraph states why you are qualified for the job (the skills, knowledge and experience you have that would make you successful on the job). The last paragraph states why you would like to work for the organization to which you are applying.
For examples and more information, you can go to the following sites.