Exchange Traded Funds are investment funds traded on stock exchanges, they usually closely track the price of a stock index or commodity price, Wikipedia expands:
An exchange-traded fund (ETF) is an investment fund traded on stock exchanges, much like stocks. An ETF holds assets such as stocks, commodities, or bonds, and trades close to its net asset value over the course of the trading day. Most ETFs track an index, such as a stock index or bond index. ETFs may be attractive as investments because of their low costs, tax efficiency, and stock-like features. ETFs are the most popular type of exchange-traded product.
The Exchange Traded Funds covered on GLDSLV.COM are ETFs which track the price of precious metals such as Gold and Silver. The site attempts to provide you will all the information you will need in order to trade such securities, including current GLD & SLV price quotes, as well as the Gold Spot Price and Silver Spot Price which the ETFs track and explanations on how the SPDR Gold Trust (GLD) and iShares Silver Trust (SLV) work.