Pull any published book off your shelf and look at the number of pages. Not just the numbered pages, account for what number the numbered pages start with, the title page, the pages with Roman numerals in the introduction, and the blanks at the end. Now divide that number by 16. You got a whole number, didn't you? That's because almost all books are printed in multiples of 16 pages, called signatures.
To understand why, take out a piece of paper. It has two sides, so it's two pages long. Fold it in half, and it has 4 pages. Fold in half again, and you have 8 pages, and in half one more time and you have 16 pages. That's a signature. If you want to make the book longer, you add another signature of 16 pages. Now, if you have kids, staple or sew the folded side, trim the other folded edges, and let them write and/or illustrate their own little book. You never know where that could lead--it certainly worked out well for the Bronte sisters (Google: Bronte Sisters juvenilia).
Usually, an author doesn't have to worry about signatures. When you deliver your final revised manuscript, the editor sends it off to their production department with a request for a target page count that is a multiple of 16. This estimated target page count is based on factors like the final word count, comparable titles, price point, and unit cost. A designer will then choose a font style and size, make decisions on leading (the space between lines) and margins, adjust features like section headers, and accommodate illustrations to fit that page count. If the target page count produces an issue (a font too small to read, for example), they'll discuss adjustments with the editor.
There are a few tricks designers can use when the page count is just shy of a signature. They may start all chapters on the right hand page to increase pages, or start chapters on the next page so some appear on the left to condense the page count. There may be blank pages at the end of the book, though publishers generally try to limit these to six. A half title page--the page with just the title that appears after the regular title page, just before the first chapter--may be added or dropped. If a book is converted from hardcover to paperback, it may be just a few pages too long for a signature break, so the dedication may be moved to the copyright page. The editor may also ask the author to create a reading group guide or Q&A to fill up the extra pages, though these are usually additions to enhance the book and offer something extra for readers.
Signatures are why it's important not to make major changes once your book is typeset, and why book contracts often have something about not making significant changes once the book is typeset. Having to typeset the book a second time not only costs money, it can throw off the schedule and in worst cases jeopardize your publication date. This is why it's important to deliver everything that you want in the book to your editor when the final manuscript is due. I found a majority of authors forgot to include the acknowledgments and dedication until the last minute (it's hard to think of all the people you want to thank when you're just focused on finishing the book and meeting that deadline!) If you really need more time, for example for an introduction someone is writing for you (or for that acknowledgments page), let your editor know it's to come and about how long it will be, so she can ask the designer to hold pages.