Paperback
Also called Softcover, softback
A book with a flexible card cover, attached to the text block by glue or sewing. The opposite of a hardback, which has rigid boards.
In practice
Lighter, cheaper and quicker to make than a hardback. The decision comes early because it changes the cover itself: a single sheet of card, with or without flaps, whose stiffness decides how the book holds up. For a thick book or one meant to last, ask to handle a paperback in the same trim size before you choose.