There is a really good explanation of the differences between a mass market paperback and a trade paperback book in the New York Times book review section today.
To read article click here
I highly suggest reading it if you have any questions in regards to the differences between the two paperback formats. I even learned some interesting paperback facts that I was previously unaware of. I have talked to many non-booksellers about the difference between a mass market paperback and a trade paperback and there is always confusion even after the conversation is over. Granted, I'm not the best at explaining things, but this also happens to be a confusing differentiation for the non-book business individual.