ETF
An exchange-traded fund (ETF) is a fund whose units trade on a stock exchange like a share. One unit can hold hundreds of stocks, a basket of bonds or gold.
SPY tracks the S&P 500, QQQ the Nasdaq-100 and GLD holds gold bars. You own units of the fund, and the fund owns the assets. Leveraged ETFs aim for two or three times the daily move of an index; because they reset daily, they are built for short holding periods.
Learn more: classes of instruments.