I often get asked this, since it is my job title. Michael Hyatt the President and CEO of Thomas Nelson has a blog and in his description of the publishing process he defines what an "acquisition editor" is and does--thank you Michael!
From Where I Sit:
Hurdle 1: The Acquisitions Editor
Acquisitions editors are the people inside the publishing house specifically charged with finding and developing authors and books that are congruent with the publisher’s mission. Over time, they have developed a “nose” for the right projects. They usually see hundreds of proposals every year. Good editors can review a proposal and decide in sixty seconds or less whether it merits further consideration. If it doesn’t, then it gets tossed into the rejection pile.
Typically, an acquisitions editor has unlimited authority to say “no.” They can reject a proposal without approval from anyone. Conversely, they don’t usually have the absolute authority to approve a proposal for publication. The most they can do is shepherd the proposal through the next step in the process.
This is why your first objective as an author is to sell the acquisitions editor. He’s the “gatekeeper” to the publishing house. If you can’t do that, you’re dead in the water. This is the one place where you have the most control. You must develop a compelling book proposal that gets the acquisitions editor’s attention. You must show that the content is compelling and there is a viable market for it.
I recommend you start with two sources: my article, How to Write a Winning Book Proposal (a PDF), and Terry Whalin’s Book Proposals That Sell. Both of these will help you clear the first hurdle.
