Show your resume to a friend, professor, a career counselor or the cognitive science undergraduate advisor. Letter of application, interest letter, cover letter, all of these are names for the same thing. A cover letter serves the purpose of introducing the reader, recruiter, or employer to you and to your resume. Like resumes there is not ONE correct way to develop a cover letter, and with time and practice cover letters become easier to write. Tailor each cover letter to the position.
Seasoned recruiters can spot a form letter immediately.
You may find that if you are applying to similar positions, you can borrow phrases and ideas from previous cover letters, but be sure to personalize each letter. Try incorporating information you know about the organization into the letter. You don't want to be eliminated from the list of potential candidates because you didn't follow directions. Be clear and concise. While there is no hard and fast rule about length, cover letters should be one page.