Not every job application asks for a cover letter, but in the cases where you need one, it can be the reason you do (or don't) get an interview.
A cover letter is a chance to show personality and humanize yourself to the person doing the hiring, or deciding who gets an interview. It's also an opportunity to point out skills that may not be evident on your resume. In addition, your cover letter allows you to address how you might acquire skills the job requires, but you don't have.
A good cover letter won't always land you an interview, but a bad one may kill your chances before they have really started. Delivering a perfect cover letter isn't hard as long as you follow a few simple steps.
1. Always personalize
Cover letters are not one size fits all. It's OK to have a template to start from, but you need to modify it for each job you apply for. At the very least, make sure you include a line that shows you thoughtfully read the job positing.
2. Address any questions asked in the job ad
If the job listing asks what your favorite animal is, make sure you answer the question. More importantly, if the ad asks about skills or experiences that are not evident from your resume, make a point of addressing how you acquired those skills. And if the ad asks for past salary information or a range desired, answer or accept that not providing the info may cause you to be passed over.
3. Show your personality
Cover letters often seem very robotic. You don't want to be too loose, but it's OK to share a little about yourself. That might be as simple as adding a line or two about your hobbies and interests, or it may be making a clever remark in response to something mentioned in the ad. Call this a calculated risk, and do be careful, but letting a little bit of yourself in could help you stand out from the crowd.
4. Make a case for yourself
If you are not an obvious fit for a job, take a shot at explaining why you think you can handle it. For example, when I first broke into business journalism, I had news writing experience, but nothing specifically in the financial space. I used my cover letters to explain why spending time running retail stores and a factory gave me a unique perspective as a business writer. (And given what I do now, it obviously worked).
5. Check and check again
The best, most clever cover letter can be ruined by a spelling mistake or poor editing. Make sure you have someone available to check your work and to make sure everything you write comes across as you intend it.
It's also smart to use the technology tools available to you. Run spellcheck and grammar checks in your word processor. These won't fix every error, and can't replace a human copy editor, but it's a good habit.
There's not one answer
Cover letters are not as rigid as resume formats. That doesn't mean you should send a flip book or a word cloud, but it's OK to take some risks. In many cases, how risky you go should depend on how directly qualified you are.
In a case where you're an obvious fit, play it pretty safe. If you're pushing to be considered for something further afield, then be a bit bolder.
However you play it, it's important to remember that your cover letter is not a throwaway. It's as important, and in some ways more important, than your resume because a bad cover letter means your resume may never get looked at.