In most situations, a cover letter should accompany your resume. However, often times it is no longer requested and many online applications do not allow you to upload the letter.
A cover letter provides you with an opportunity to highlight your interest in the organization and position, what you know about the organization and why you want to work for for the organization.  You need to demonstrate that your skills are a good match. It does not need to restate what is on your resume, but it should mention your soft skills that are not on the resume, e.g., communication and time management skills. Your one page cover letter should be addressed to a specific person. "To Whom It May Concern" is not an acceptable salutation. Research may be required to identify the appropriate person to address the letter to. This is where networking with HMC alumni can be helpful.  If you still cannot find a specific person, address it to: Dear Hiring Manager. The cover letter also demonstrates your writing ability, so you might want it critiqued by staff in the Office of Career Services or Writing Center.

View cover letter samples in the HMC Career Guide. You can also pick up a Cover Letter Guide in the career center.