Paperback may refer to a kind of book binding by which papers are simply folded without cloth or leather and bound - usually with glue rather than stitches or staples - into a thick paper cover; or to a book with this type of binding. (Contrast hardbound or hardcover.)
One of the first publishers to exploit the potential of paperbacks was Penguin Books, and since then paperbacks have become commonplace.
Paperbacks include cheap mass market paperbacks, in the standard "pocketbook" format generally printed on newsprint or other low quality paper, which will discolor and disintegrate over a period of decades, and more expensive trade paperbacks in larger formats printed on better quality paper, sometimes acid-free paper.
See also The Beatles' song, "Paperback Writer".