Use the search function before asking stuff. This is a VERY commonly asked question.
Try it. And if you still don't have your answer, I'm sure someone will provide you with one.
EDIT: Simply put, the MidGrade and regular UltraEdge are the same wall thickness,
and HeavyGrade and UltraEdge HeavyGrade are the thicker, more durable (some say virtually unbreakable) ones.
That's how I've understood it.
Well put Nyte. Nothing against newbies, but they should realize there's a search function that you can use to avoid making highly redundant threads. Having said that, Nyte is quite correct about the durability - the Ultraedge and regular heavy grade blades are thicker walled and meant for heavy dueling, whereas the midgrade and Ultraedge are for moderate dueling. Really they're both quite durable, so go with what you want. "You must do what you feel is right, of course..."