I am having trouble trying to figure out the difference between a mass market paperback book and a paperback book.  I have seen books identified as one or the other and when I have ordered them I can not tell the difference between them.  They both look the same when I have them in my hands.  How can I tell the difference when ordering them???

A mass market paperback is what used to be called a "pocket" paperback - that is, it was narrow enough to fit in a pocket (although obviously many are not).

A trade paperback is the same size as the hardcover version of the same book.

It's almost impossible to tell when ordering a book here which it is going to be. A few paperbacks do show that they are mass market, but the site never differentiates for trade PBs. You'd pretty much have to look the ISBN up on Amazon.com to find out which it is - you can usually figure it out there.

A "paperback" listing here may be a mass market paperback or a trade paperback.  I don't know of any way to know in advance which one it is, other than sending a PM to the sender and asking.  The system doesn't really seem to differentiate between them, so unless you are looking for a particular binding I wouldn't worry about it. 

Basically, any book that's about 7 inches tall is a mass market paperback.  More than that, you've got a trade paperback.  Here's a Wikipedia page talking about different types of paperbacks: http://en.wikipedia.org/wiki/Paperback#Mass_market_paperback

Oh - I thought of a way to tell.  Look up the book at Amazon (when possible) and check out the dimensions.  As I said, if it's around 7 inches tall by around 4.25 inches wide, it's a mass market.  If it's taller and wider than that, it's a trade.

When listing a book, if it states the edition is MMPb, then you want to make sure it is the smaller sized paperback. if it states Trade PB, then you want to make sure it is the larger sized pb. If it just states paperback, then you are safe with either one. It makes a difference when you are trying to put a matched set together. for instance, I was collecting the Jan Karon Mitfrod series and just assumed they were all Trade paperbacks. I was disappointed when i received a couple of them in MM size, but it was my fault for making assumptions . . . I can relist them, and probably will, and next time I'll verify the edition more carefully!