Paperback, or as it's often called, mass market paperback, is the mainstay of the publishing industry. Mass market titles generally appeal to a broad audience and readily available in a variety of retail outlets. Some books are published as paperbacks originally, others are printed in paperback following a hardcover release.
The size of the mass market paperback is usually 4-1/4 inches x 6-3/4 inches which is a smaller format than the trade paperback. The price of the paperback is also less than the trade paperback. For purposes of calculating the bestseller status of a book, hardcover and trade paperback sales are tracked separately from mass market. In addition, often only hardcovers are given review space in some publications.