A query letter is sent to an Editor or Agent to solicit the sale of your book. This is not a blurb. This is not a synopsis. This is a letter explaining you and your book.
Things to think about: Simple is better. Clear and to the point. Editors and agents don't have time to dig through a three page idea about your book. Be concise. Be bold. Each day an Editors and agents email may have hundreda to even thousands of submissions for them to sift through. Think about what would make your query stand out to them. What is special about your story? Get more bang for your buck with your word use. Just like in your writing cut the adverbs and be straight and to the point about what you have written and who you are. Make sure before you submit to an editor or agent that you research so that you know what they are looking for. If your story or series fits that then word your query with items that attract attention because it is a fit. Don't bother submitting something they aren't accepting. Find the right editor or agent. Study up on Editors and Agents like a stalker so that you will know just how to get your story across to them. And if you are planning to send out a generic letter to 50 publishers all at once, that may not be the best plan. You may get responses and even requests but is that a right fit for you and your story? Do your homework. You can start out by introducing who you are and maybe even mention you understand the editor is looking for the type of stories that you write. Explain that story. Stick to 3 to 5 paragraphs of 3 to 4 sentences. No more than a page. And for goodness sake use spell check prior. You may want to write a short biography about you and mention you attached it for their review. If you have been published before by all means share your accomplishments but be brief. It's OK to mention what you do for your day job in your closing to add a little color to your presentation of yourself. You can read examples of query letters online for examples. Remember though that you need a polished manuscript to submit. And I mean polished. Not half done and unedited. And don't forget to thank them for their time. Read up on this.