Paperbacks are books bound with flexible paper covers; they usually have adhesive rather than sewn binding, and so have a shorter lifespan than hardcover books. Many paperback books are less expensive editions of hardcover books, though today some books are first, or only, published as paperbacks; Kurt Vonnegut, Jr. was apparently the first author to have his books appear only in paperback. Most paperback books are cheap mass market editions; there are also quality paperbacks or softcover books which are larger and more expensive, and are sold in bookstores as opposed to drugstores and airports.
Paperbacks might seem to be something thoroughly modern example of disposable goods, a product of cheap paper and mass printing techniques. But in fact paperbacks have existed for a long time, though not in a form we would easily recognize today. The very earliest texts were written out by hand onto paper made of papyrus or other reeds or cloth; bound only with a tie, or even folded up, they could, I suppose, qualify as paperbacks. As paper making developed in complexity, manuscripts were still written by hand, but might be bound in heavy wood covers wrapped in leather inlaid with lettering and sometimes embossed with illustrations. The big revolution came with the invention, in the 16th century, of the printing press; for the first time books could be produced in large numbers, and at a much lower cost, than had been true when books required months of painstaking copying. Bindings too became lighter and less costly, with pasteboard - a kind of heavy paper - being the covering of choice. These, then, were the first books with paper covers, though not flexible ones.
By the 18th century true paperbacks were being produced; they were small books or pamphlets known as chapbooks which became extremely popular in Europe. Because chapbooks were inexpensive and easily available - they were sold by travelling salesmen known as chapmen, as well as at local stationery stores - they were a good way for an author to boost sales. Many told fantastic tales - akin to modern comics but with more words and fewer drawings - but serious writers published chapbooks as well. John Stuart Mill, for example, wrote many chapbooks, and Henry Fielding's Tom Jones first appeared in this form. Similar books began to be published in the United States in the 1830s; they were pirated editions of English titles, sold first as supplements to newspapers and later as small-sized books. They were exceedingly popular for some decades, until the Copyright Act of 1891 cracked down on pirating. Paperback production would languish until the English press Penguin reintroduced them in 1935, and since that time they have enjoyed another meteoric rise in popularity.
A useful history of paperbacks can be found in Hans Scholler's "The Paperback Revolution", in Essays in the History of Publishing (Asa Briggs, ed.). Also fun is Quality Paperback Book Club's overview of the history of the paperback at
www.qpb.com/oascgi/timeline_home
QPB publishes "radical" books which deal with topics like homosexuality or emancipation which conservative houses might not touch, so their timeline includes fun information like the first book banned as obscene in America (1749, Fanny Hill, of course), and the first book published by an African American woman (1773, a book of poems by Phillis Wheatley). (update April 13, 2003: sadly, QPB's timeline seems to no longer be online.) Also useful, though less saucy, is Bruce Jones' longstanding and richly documented website on the history of the printing press and its impact on the world at
communication.ucsd.edu/bjones/Books/booktext.html | 
