Most people I know hate cover letters to the extreme that they often opt out of them. But, cover letters are important and there are a few rules you should follow.
Q. Do I always need a cover letter?
A. No, only if you really want the job. Otherwise, apply away without one, but don't expect anything.
Q. Is there a specific format I should stick to?
A. Yes and no. When applying to a professional or business related job, absolutely. State your name, the job you're applying for, and throughout the letter, explain why you're the best candidate for the position. For more creative jobs, you have a bit more freedom to spice your letter up, but you have to understand that not everyone may love your creative liberties.
Q. Can I use the same cover letter for multiple jobs?
A. You can use the same basic format, but you really have to edit and tweak (even if you're applying for the exact same job title). Each company is different and looking for slightly varied qualities. Be sure to always double check that you send the correct cover letter.
Q. Who do I address the letter to?
A. This is a key question. You should really do your research and find out who the hiring manager of the company is. If you're unable to or if their are multiple people hiring, you can always write "Dear Hiring Manager". It definitely shows a lot when you put the extra effort into finding out who to address the letter specifically to.