Five-ply stainless steal?

[FoxyWiles]

What does three and five ply stainless steel mean? How do they stack up against a regular 18/10 stainless steel item?

I'm asking because I'm in the market for a roasting pan - one that will last me as long as possible that isn't copper. I found this:
http://www.amazon.com/gp/product/B000...