An ETF is an Exchange-Traded Fund, an alternative to a mutual fund.   One share of an ETF represents a slice of a basket of company stocks, just like a share of a traditional mutual fund.  However, an ETF is bought and sold on the open market, like a company’s stock.  That is, instead of buying it directly from a mutual fund company, the investor buys it through a stock brokerage firm, and any fees are like those that are involved in buying or selling stock. 

ETFs were only introduced a few years ago, whereas mutual funds have been around for decades.  Most of this website is written in terms of traditional mutual funds.