According to John Reed with Robert Half Technology, when speaking to relevant accomplishments, "you are demonstrating to the employer that 'I've read your job description. I think I understand what you are looking for. I can do that job and do it very well and here's why.'"
Choosing your accomplishments to add to the cover letter smartly will help demonstrate that you've dealt with the same types of problems they are currently dealing with and more importantly how you can help.
For step-by-step details on how to create an effective cover letter, see 13 Tips for Writing an Effective Cover Letter.