Mass Market paperbacks tend to be reprints of hardcover bestsellers plus genres like mystery, thriller and romance, trade paper tends to be more literary or serious fiction.
"Mass market" were the original paperback books sold in racks at places like drugstores and grocery stores.
"Trade paper" is closer to the size of a hardcover book, costs more and is usually found in bookstores, although this distinction is not as clear now that there are more places that sell both like Walmart.