XAUxxx is spot gold price. Quoted in US dollars (USD) or Euros (EUR).
Basically it is traded like another pair in forex.
Two major gold indices dominate the market: the Philadelphia Stock Exchange XAU and the AMEX Gold BUGS Index (HUI). The major difference between the two is that the BUGS index is made up exclusively of mining stocks that do not hedge their gold positions more than a year-and-a-half into the future. This makes the BUGS Index much more profitable than the XAU when gold prices are rising, but can also compound its losses when gold declines.
BUGS is an acronym for Basket of Unhedged Gold Stocks. The index was introduced on March 15, 1996 with a starting value of 200.