(00:04 on Wed 16/Aug/06)There's very little difference, I think, except that when I buy 'mass market paperbacks' in at work, they're usually of a slightly smaller format than the standard paperback (std paperback approx 13cm x 19 cm). The smaller format seems to be more common on the US market. The paper quality is similar, but the print is smaller and they do seem to have a certain 'cheapness' about them.
What you might also come across is the trade paperback, which is a sort of cross between paperback and hardback. A posh paperback, if you will, that the publishers can sell for more money than an ordinary one.