Yes and no. There is some overlap, but the differences are significant.
A one-sheet gives writers a document for talking points about a project at a conference. The one-sheet can help authors be sure they convey the information they want to the editor or agent without forgetting anything critical. In turn, the one-sheet gives the editor or agent a memo of sorts to recall your pitch after the conference. This is one reason why an author photo is essential. Otherwise, the one-sheet includes information such as the book theme and brief plot summary, contact information, and sometimes another visual to make the page pop. One-sheets are often colorful and intended to grab attention. However, they are only a tool. The author’s professionalism and talent are key.
A query is a letter an author sends an agent or publisher. It is not meant to be eye-catching in a visual manner, but attention-grabbing because of the information it conveys. Professional stationery is great for hard copy, but a query letter should not include an author photo. The recipient can visit your web site for visuals. A query always includes the author’s contact information, web site address, book idea, a brief overview of the author’s publishing history and a couple of key marketing helps. Be brief by choosing only the best information to convey to convince the agent or editor that your proposal is a must-see. The standalone query letter should end with a request to submit a complete proposal.
A query letter that introduces an attached proposal is just that — an introduction. This letter offers the author’s contact information, book theme, and brief summary and perhaps a piece of information so irresistible that the recipient must stop everything to read your proposal.
Wise use of these tools will accentuate your efforts to find just the right home for your work. Add these to writing that sparkles, and you are on your way!