Books for small children that are made of heavy cardboard instead of paper so as to be difficult to up and destroy. The better ones also have a super-duper binding for the same reason. Some cheap ones have horrendous binding that falls apart instead.

They usually are smaller in size than regular books, while being thicker than them. That's why some people don't like them, because it can be hard to fit on your bookshelf.

Sometimes they're also abridged, usually without any warning on Amazon of this fact - I always mark as helpful any comment that tells me that a book has been pared down from its original version!

All the books I've entered so far (except a few of them) have been board books.