The "petrodollar" is the currency (us dollars) which is used to buy and sell Saudi oil.
http://en.wikipedia.org/wiki/Petrodollar
The only difference between a petrodollar and a regular dollar is that the petrodollars don't really make it back inside the US and therefore aren't a normal part of circulating currency.
Makes sense. The King's and Prince's drift around in their gold plated cars while the people eat shit. I wonder how much of the 18 trillion debt has flooded their markets?