Don't take a shortcut in not sending a cover letter with your application. This is a big mistake. A well-written cover letter tells the employer that you are interested in the position. Unless you're told not to send a cover letter, you should always send one.
The next big mistake that too many people make is to send a generic cover letter. This mistake is almost as bad as not sending a cover letter at all.
When you apply for a position, you have to ask yourself whether you want the job. If you don't want the job, don't bother applying. If you want the job, do your best to get it.
It is preferable to apply to less positions, but spend more time on each application, than to send out a bunch of applications indiscriminately.
This series on cover letter writing will help you write a better cover letter:
As you write your cover letter, if you're wondering whether to use bullets, paragraphs, or tables, read this article: Choosing the Best Cover Letter Format: When to Use Bullets, Paragraphs, and/or Tables.
Nowadays, many job applications are sent by email. This article discusses the differences between a conventional cover letter and a cover email: Submitting a Cover Letter by Email: What to Do and How Is It Different?
If you want to see examples of cover letters, see our database of cover letter samples.