Let me say that I've done more hiring and firing in my days than I care to admit. And although Human Resource Managers and recruiters all have their own way of doing things, here was my approach to looking at new applicants...
A Good Resume Invites Recruiters to Read Your Cover Letter
I always looked at person's resume first. I quickly breezed over qualifications and employment history. If they looked promising in these areas, then I would look at cover letter.
The cover letter gave me further insight into person, e.g. how organized they are by examining presentation of letter, whether or not they had taken time to find out anything about my company, whether or not they have had or do have any ties to company, whether or not they have specified job they want, will they travel, and so on.
What all this means is you need both a resume and a cover letter. The meat is in resume. I wanted facts and I wanted 'em fast. If a person looked promising from resume, then I read cover letter, and then I call 'em in for an interview. That's where rubber meets road.
The Goal of a Cover Letter
So, this brings up your goal in creating a cover letter ...
... to introduce yourself to company and to sell yourself.
If you do this well enough, you are at top of list for an interview, assuming you are qualified for position.
Creating a Strong Cover Letter
1. Make it short. Definitely no more than one page. Half a page is better. Two paragraphs is even better. If you can whittle it down to that and make your case succinctly, then you are proving yourself to be a great communicator. That's a huge bonus in my book. 2. Be professional and concise. Never try to be humorous. Keep your tone on a "business only" level.
3. Always put yourself in best light and never give hints that you may be underqualified, even if you think you are.