Basically this means know to whom you're sending the letter. Address your letter properly. I know of a prominent agency in New York City. Both agents are women. Also, don't assume a familiarity.
Business is still very conservative.
Your salutation should be to Mr., Mrs., or Ms. If you're unsure, use the recipient's full name. Your response from the agent or publisher will be according to your letterhead or how you sign your letter if it's not a form letter. You can find more information regarding what should be in the body of your query letter in the body of my talk to the North Texas Professional Writer's Association. If your material is requested, your cover letter should follow the same rules as above.