There are two basic types of cover letters:
A letter of application is used when applying for a position that has been posted and is currently available. The letter should state how your education, skills, and experience match the qualifications listed in the job posting. To write an effective cover letter, examine the job posting and customize the letter for the specific job and company. Instead of repeating everything on your resume, focus on highlighting your main qualifications and drawing connections between your experiences and what the employer is looking for.
A letter of inquiry, or a prospecting letter, is sent to express interest in working for or interning with a particular employer when there is not an advertised job opening. Summarize your qualifications and highlight how you contribute to the company. You must communicate why this company interests you and why you want to work for them, so it must also be customized for the specific company. A prospecting letter is most effective when you can reference how you heard about the company, whether it is through one of your contacts or your research. However, not all employers accept unsolicited letters and resumes.