Some smart people came up with the idea of the ETF ("exchange-traded fund"). An ETF trades just like a stock. You can buy or sell it all day long in your brokerage account. Each ETF represents a certain index. So the ETF for the S&P 500 trades under the ticker SPY. The ETF for the DJIA trades under the ticker DIA. And the ETF for the Nasdaq 100 trades under the ticker QQQ.
