What constitutes a book being a MMP? Is it the author? ARe most Nora Roberts, John Grisham etc. considered Mass Market Paperbacks? Is there content that is edited out? I get that there are mass quantities of books printed, but other hten that, I'm a little lost. Thanks!

A mass market paperback are the smaller ones (like the authors you mentioned) - think the ones you pick up at the grocery store. Trade paperbacks are bigger.

The content is the same - its just that the size is smaller and therefore, the book is cheaper.  Some people dislike MMPs because they are so small, but others like them because of the cost.