You're not the first person to ask this, Rose, and you won't be the last! What confuses writers is the difference between a query letter and a cover letter.
When you're sending a query letter, it's a cover letter and mini-synopsis all rolled into one. You're asking for permission to send more information.
If the agent/editor has requested a synopsis and chapters, however, you're skipping that initial step, but that doesn't negate the need to send a cover letter. A cover letter should include the following information:
Your name (with a Ms. or Mr. if it's a unisex first name -- like Jody or Lee)
Your Address
Your E-mail Address (if any)
Your telephone number (daytime)
The name and address of the person you're contacting
The name of the book
The length of the full manuscript
The reason why you think it would be a good book for their line or imprint
A thank you directed to the reader for their time.
A request that they look at the full manuscript.
Your name and signature
Does that help?
