The purpose of the cover letter is to briefly introduce yourself so that prospective employers will look at your resume. After they look at your resume, they may decide to call you in for an interview. After the interview, you should send them the thank you letter for inviting you to the interview. These three documents should be generic in nature; they should be templates that outline the ideas that you wish to communicate. Then, when you need to deploy them, customize them to fit your particular job application.