What’s the difference between a query and a cover letter?
For most practical purposes, they’re the same but with a little twist.
In a query, you’re asking if you may submit your manuscript. The query letter includes info about your book: genre, word count, a brief description of the plot and the request to submit the manuscript. Depending on the publisher’s submission guidelines, the query letter may be accompanied by an outline/synopsis and the first few pages/chapters—or it may be a stand-alone submitted all by its lonesome self. (More posts about querying.)
You use a cover letter after the publisher/agent has agreed to read your manuscript. It goes on top of the hard copy manuscript when you mail it or in the body of the e-mail to which your manuscript is attached. The cover letter includes info about your book: genre, word count, a brief description of the plot and a thank you for reading the manuscript.
It’s important to read the publisher’s guidelines. In most cases, if you want to submit a picture book you need to send the whole ms with a cover letter. LDS publishers tend to want the whole ms if it’s fiction. Magazines also tend to want the ms as opposed to a query. However, some publishers will only consider a query and will not accept unsolicited manuscripts. Checking the guidelines will save time, money, and aggravation for the author and the publisher.