A paperback is a book which is bound in stiff paper or flexible card. A book which is not an e-book is either a paperback or a hardback. The latter is a book which has a hard cover such as cardboard, heavy-quality paper or a material known as "buckram."
Publishers usually launch a book with a hardcover and wait approximately 12 months before releasing the paperback edition, although ultimately the time frame is determined by the number of sales. The ISBN reference is followed by "Hbk" if the book is published in hardback and "Pbk" if the book is published as paperback.