Recently I read a fantastic announcement that an online acquaintance had received a contract for a "chapter book". I was really excited for her (the contract is with Disney!), but had to admit I hadn't a clue to what a "chapter book" was.
Enter my gal friend Wiki (Yes, Wikipedia is female. She obviously knows everything!):
A chapter book is a story book intended for intermediate readers, generally age 7-10. Unlike picture books for younger readers, a chapter book tells the story primarily through prose, rather than pictures. Unlike books for older readers, a chapter book contains plentiful illustrations. The name refers to the fact that the stories are usually divided into short chapters, which provide children with opportunities to stop and resume reading if their attention span is not long enough to finish the book in one sitting.
This got me to thinking (I know you smelled the smoke) "What are the different types of children's books?" With a bit more digging I found a brief listing on the
Find Me An Author site.
Picture books
In its broadest definition, a picture book is a book in which the illustrations play a significant role in telling the story. Traditionally, picture books (also called "picture story books") are 32-page books for ages 4-8 (this age may vary slightly by publisher).
Early readers
Also called "easy-to-read", these books are designed for "emergent readers" - children who are just learning to read on their own. Most of these books are intended for primary grades i.e. children just starting to read on their own (age 6-8). They have color illustrations on every page like a picture book, but the format is more "grown-up" -- smaller trim size, sometimes broken into short chapters.
Chapter books
For ages 7-10, Chapter books are transitional books that help children move from early readers to full novels. Most chapter books deal with contemporary situations that are familiar to the readers, and are often humorous books.
Middle-Grade Novels
This is the golden age of reading for many children, ages 8-12. A novel, unlike the picture book, relies entirely on text to tell the story. Kids get hooked on characters at this age. Fiction genres range from contemporary to historical to science fiction/fantasy.
Young Adult
Young adult novels, also sometimes called teen novels, are generally read by children anywhere from age 12 upwards. They are longer books, often 50,000 words or more. Most are contemporary stories, and often involve contemporary problems, including drugs, sex, and peer pressure i.e. popular themes are usually relevant to the problems and struggles of today's teenagers, regardless of the genre.

By the way, keep an eye out for
Iridessa and the Fire-Bellied Dragon Frogs (tentative title) by Nnedi Okorafor .