A cover letter is often required for an internship or job. Even if it isn't required, it is always a good idea to include one.
How is a cover letter different from a resume?
A resume is an overview of the facts: where you went to school, what jobs you have had, what skills you have. Although resumes can be adjusted to reflect the needs of the specific position you are applying for, the basic information remains the same.
A cover letter is much more personal and tailored to the specific position you are applying for. Discuss why you are interested in a particular position, what skills you bring that make you the perfect candidate, and why you would be a good fit for that particular employer. Make it a formal business letter, but it is ok to let some of your personality shine through. This is a chance for the employer to get to know you.
You should write a unique cover letter for every position. Employers can spot a "canned" or generic cover letter a mile away and they do not like them!
Cover Letter Sample Note how this writer has addressed the requirements in the position description.