The cover letter is a sample of your written work and should be brief (preferably one page), persuasive, well-reasoned, and grammatically perfect.
A good cover letter:
Tells the employer who you are (e.g., a first-year student) and what you are seeking (e.g., a summer intern position);
Shows that you know about the particular employer and the kind of work the employer does (i.e., civil or criminal work, direct client service, impact cases, antitrust litigation);
Demonstrates your writing skills;
Demonstrates your commitment to the work of that particular employer and conveys that you have something to contribute;
Shows that you and that employer are a good fit; and
Provides contact information so that the employer can get in touch with you by email, telephone, and mail.