Exchange Traded Funds (ETFs) are funds that track market indices in order to mirror the return of a given asset class or basket of underlying securities. They are traded on exchanges in the same way that a stock or bond would be traded, making them more liquid and tax-efficient than mutual funds.
Examples of ETFs include whole market tracking funds, S&P500 funds, and funds that track other major asset classes. ETFs may also be used for thematic investing (e.g., tracking tech stocks or socially responsible indices), factor investing (e.g., volatility or any other attribute of collections of securities), or tracking a variety of other assets in a liquid way.