Irish moss is a clarifying agent. Drop it in the last 5 minutes of the boil and it wll help clear your beer. Of course letting it sit in the primary for a month or using a secondary will also clear it up nicely. Not sure about Sweet moss...
Irish moss is actually an algae that grows on the coast of Great Britain and North America. It's purpose in beer is to help coagulate proteins in the boil which results in a cleaner finished product.
I was looking at some misc brew gear on CraigsList. The guy wanted way to much for it, so I didn't buy it. But one of the items he was selling was "Peat Moss". I played dumb and asked him what the peat moss was for.
He knew it's use, just not it's name.