When you make a book by folding a sheet of paper in half, and then in half again, you've made a quarto. Bestselling books are not usually published as quartos these days, but you might find an old Shakespearean text this size.
Use the noun quarto to talk about a book that uses this same basic folded design. Each page of a quarto is one-fourth the size of the original sheet of paper, which is where its name comes from: the Medieval Latin in quarto means "in the fourth (of a sheet of paper)." Quarto also refers to a book of this size, or a piece of paper that measures ten by eight inches.