It is proper job seeker etiquette to preface your application to an advertised job posting with a cover letter.
Some advertisements contain explicit instructions on what should be contained in the cover letter. If so, ensure you follow their instructions or you run the risk of your resume being automatically dismissed.
If there are no instructions, here are my recommendations for a cover letter:
1. Clearly (often bold or as a header) identify the job you are applying on.
Use the advertised job title, a reference number, and any other detail that links your application to an actual job. A busy recruiter receives hundreds of applications a day & if she can not link a resume with a job, the resume may sit idle.
2. In the first short paragraph, indicate how you became aware of the job. Say where you found the job advertisement or who referred you to the job.
3. In the second paragraph, use your "elevator talk". Concisely, summarize your potential value to the company. A previous post provides some pointers on creating your quick value proposition.
4. In the third paragraph, address advertised duties with some detailed past actions and results. Here, I recommend a bullet list of about 3-5 facts that prove you can do the job. Be specific, use industry jargon and business names. This all helps a reader link trust what you are saying more since your sentences tell specific stories. If a job advertisement indicates that you will manage the corporate blog, in my cover letter a bullet of mine would be: Utilizing blogger.com, run a blog for career transition which has over 800 hits from 5 countries in its first two weeks.
5. In the concluding paragraph, actively ask for a meeting. Possibilities include asking for a phone conversation (please call me at 780.... as I would love to talk to you about this opportunity), an in person meeting (I am free next week and work very close to your office. If you have some free time, send me a quick email and I can meet you). Provide any contact information that may not be in your header and/or clarify the best way to contact you.
After all, a great cover letter will not get you the job. A good to great cover letter will just get your application read, which may get you and interview, which may lead to them checking your references and credentials, which then may get you the job! Good luck