In bookselling, trade paperback is any paperback book that is not a mass-market paperback. They tend to be larger, more expensive, and, in theory, of better quality than mass-market paperbacks. They are also more likely to have illustrations or photographs. Often a distiction will be made between a 'trade paperback' and an 'oversized paperback', a paperback too big to fit upright on a standard shelf.
This is one step down from trade hardback, about one-half step up (in my own estimation and experience) to bookclub edition hardcovers (depending on the bookclub, of course), and one step up from the above mentioned mass-market paperbacks.
See also, Softcover, paperback.