Refers to a binding method for publications with page counts of over 48pp (in general terms).
A paperback is most commonly bound using a perfect binder, which glues together single sheets by scoring the spine edge of a book block and applying a thick layer of glue before placing a paper cover around it.
A paperback is a cheap binding method, made to emulate a section sewn book block.
A paperback is often printed on low cost, commodity paper stocks which often fox quickly and are often bound against the grain as a further cost saving measure.