A cover letter is an extension of your resume where you expand on the knowledge and skills you developed over the course of your past experiences rather than just a summary of it. This is where you provide an in-depth background to the bullet points on your resume and discuss how your experiences have prepared you for the position you're applying for.
Generally speaking, your cover letter should consist of three paragraphs and address the following points.
Paragraph #1 - Introductory statements that should be no longer than three to four sentences and addresses the following:
a) Who you are (school, background, etc.)
b) How you learned of the opportunity
c) What attracts you to the company (ex. culture, values, the people, opportunity, etc.)
Paragraph #2 - This is where you discuss your background and should be your longest paragraph and written in the following order:
a) One sentence that discusses concretely a specific past experience such as what you are currently doing, a relevant internship you did, or a skill you offer
b) 1-2 sentences immediately following it demonstrating what you have learned above and how it prepared you for the job you are applying for.
c) Repeat steps A&B
Final Paragraph - Bringing it all together
a) Clearly state what skills or qualities you will bring to the company
b) Clearly state how the company will benefit having you as an employee
I would highly recommend checking out this website, http://member.collegejoboffer.com/info
for more tips, advice and strategies on how to craft a winning cover letter as well as everything else necessary to succeed in landing job offers as I was able to land multiple offers by incorporating and applying what I had learned to the interview/recruiting process.
Best of luck! Feel free to reach out to me should you have any additional questions.