XAU is the Philadelphia Gold and Silver Sector Index
HUI is the AMEX Gold Bugs Index
GLD is a gold ETF that (supposedly) holds real bullion
SLV is a silver ETF that (supposedly) holds real bullion
CEF is the Central Fund of Canada which (most likely) holds about a 50/50 mix of gold and silver (http://www.centralfund.com/)

GLD and SLV are fine for gaining exposure to the precious metals sector – I trade in and out of them from time to time – I feel safer about CEF and consider it a long term holding – if you open a brokerage account denominated in the Canadian dollar and buy CEF in that account you gain exposure to the precious metals and the Canadian dollar while reducing your exposure to the US dollar

XAU and HUI are indexes so we can’t buy them directly – there are options available on both indexes – Adam Hamilton at Zeal (www.zealllc.com) prefers trading NEM stock (Newmont is one of the major gold producers) as a synthetic option on the HUI index

There is a fair amount of controversy over whether GLD and SLV actually hold the metals they claim – some of their paperwork is vague as to subsidiary entities being allowed to hold bullion futures positions vs actual bullion – “where there’s smoke there’s usually fire” seems like a good attitude to take in this case – use GLD and SLV as trading vehicles (I hold them in my IRA currently along with CEF) but also gain more secure exposure to the precious metals via the Perth Mint Certificate Program or your local coin shop