SPDR is a short for Standard & Poor's depositary receipt. It is an exchange-traded fund (ETF) managed by State Street Global Advisors that tracks the Standard & Poor's 500 Index (S&P 500). Each share of spider contains one-tenth of the S&P index and trades at roughly one-tenth of the dollar-value level of the S&P 500. Spiders can also refer to the general group of ETFs to which the Standard & Poor's depositary receipt belongs.
When talked about it is often called Spiders. It is traded on the AMEX with the ticker SPY. It can be bought and shorted like a regular stock and useful for index investing or a hedge against the general market.