SPDR, which stands for Standard & Poor's Depositary Receipts, is a type of exchange-traded fund (ETF) that tracks the performance of a specific index, such as the S&P 500. It allows investors to buy shares that represent a portion of the underlying assets, making it easier to invest in a diversified portfolio without purchasing individual stocks. Launched in 1993, SPDRs are managed by State Street Global Advisors and are among the first ETFs available in the United States. They provide a cost-effective way for investors to gain exposure to various sectors and asset classes while maintaining liquidity, as they can be traded on stock exchanges like regular stocks.