A children's book format. Board books are much sturdier than regular paper picture books. What they're made of is sort of a cross between cardboard and thin sheets of pressed wood. There are some board books that were board books before they were anything else. But most of them are adaptations of already-existing picture books. Most of Eric Carle's work is available in board book format, as is Peggy Rathmann's, and Margaret Wise Brown's. This format is smaller than the original - usually about 1/4 the size - so you get a compressed version of the original artwork, which may bum you out when it comes to Eric Carle. Still, board books stand up well under biting and throwing, and as such are wise choices for babies and toddlers.