Spiders - SPDR

What Does Spiders - SPDR Mean?
A short form of Standard & Poor's depositary receipt, 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.

Investopedia explains Spiders - SPDR
Spiders are listed on the American Stock Exchange (AMEX) under the ticker symbol SPY. By trading like stocks, spiders have continuous liquidity, can be short sold, bought on margin, provide regular dividend payments and incur regular brokerage commissions when traded. 

Spiders are used by large institutions and traders as bets on the overall direction of the market. They are also used by individual investors who believe in passive management (index investing). In this respect, spiders compete directly with S&P 500 index funds.