When I tell friends that I've written a children's book, but that it's novel-length, they sometimes look confused. But once I mention Roald Dahl, they instantly get it.
Of course, almost everyone knows what a picture book is and I think, at this point, YA (Young Adult) has made enough of a splash that most people know what it is, too. But there are books that fall in between that are called terms most people don't hear every day. For example, I am a Middle Grade (often abbreviated as MG) writer.
Here is a handy cheat sheet breakdown of the five levels of children's books:
Age Range: 0-2
The Skinny: These books are made to chew on as well as to read. They often involve bright primary colors, very few words, and sometimes even textures.
Examples: Pat the Bunny by Dorothy Kunhardt; Where is Baby's Belly Button by Karen Katz
Picture Books (PB)
Age Range: 0-5
The Skinny: Pretty self-explanatory. These books are usually under 1000 words, filled with full-color illustrations, and are read to kids because they are too young to read on their own. The illustrations are often equally as important as the text.
Examples: The Cat in the Hat by Dr. Seuss; The Very Hungry Caterpillar by Eric Carle; Knuffle Bunny by Mo Willems
Age Range: 5-8
The Skinny: These are often the first books that kids are reading by themselves and focus on teaching kids how to read as well as telling a story. They involve short chapters and, often, larger type. They also usually have quite a bit of interior illustrations though not as many as picture books.
Examples: Frog and Toad Are Friends by Arnold Lobel; Amelia Bedelia by Peggy Parish; Junie B. Jones by Barbara Park
Middle Grade (MG)
Age Range: 8-12
The Skinny: These books are usually children's first introduction to full-length novels. They often involve faster-paced plots and more in-depth characters than either picture books or chapter books and focus primarily on story development.
Examples: Charlie and the Chocolate Factory by Roald Dahl; Harry Potter and the Sorcerer's Stone by J.K. Rowling; The Graveyard Book by Neil Gaiman
Young Adult (YA)
Age Range: 12 and up
The Skinny: These books almost always involve teenagers as protagonists and can deal with a lot heavier and more adult issues than the other levels of children's books.
Examples: Twilight by Stephenie Meyer; The Absolutely True Diary of a Part-Time Indian by Sherman Alexie; Hunger Games by Suzanne Collins
Now you can use in-the-know acronyms like PB, MG and YA, too. So go forth and talk children's books!