This has been taken from an article called Picture Books 101 by Michael Hingston. His article was based on a presentation by Alison Hughes.

“Format and terminology:

• Jacket: The loose sheet folded around the book’s cardboard cover. The two parts of the jacket that get folded into the book are called the “front flap” and the “back flap.” The front flap usually describes the book and has marketing text. The copy on the back flap is usually information about the author and illustrator.
• Ends or end sheets: These are the pages that are glued to the cover of the book (not usable for text, but sometimes have illustrations on them).
• Book block: The actual pages of the book.
• Signature: Printers set bundles of paper sheets into the cover in sets of four. Once bound in, and counted front-and-back as pages, those four sheets make 16 pages. A 32-page book has two signatures of paper in it.
• Spread: An illustration that spans two open pages in a picture book. Usually combined with single-page illustrations within the book, depending on the needs of the story or the impact desired.
• Gutter: The dip in the middle of the book where the pages are bound. Editors and illustrators want to avoid text and illustration being lost in this area.
• Page-turn: A technical term denoting the breaks or divisions within the story as pages are turned. Page-turns can be used very effectively as part of the storytelling to build suspense, surprise, fun and emotional or visual impact. They’re important to think about when you’re writing and editing the story. The classic example of a book using page-turns to great effect is The Monster at the End of This Book.
• End Sheets: Once bound into the book, page 1 is not part of a two-page spread, and neither is page 32. Those are end sheets. In a 32-page picture book, you don’t actually have 32 pages for your story. You really only have 24–28 pages, since 4–8 are used for the book ends, copyright and title pages.
• Back matter: This is factual, interesting material that is appended to the back of a fiction book. It elaborates on some aspects of the book, and should be a natural fit for inclusion. It gives parents/educators a little more room for starting conversations.
• Dummy: This is when a writer/illustrator plots out a picture book in an actual book layout (see sample layouts). You don’t have to do this, but some people find it helpful. Depending on the editor/publisher, they may or may not want to see a copy of the dummy you create.”