DEFINITION of 'Exchange-Traded Fund (ETF)'

An ETF, or exchange-traded fund, is a marketable security that tracks an index, a commodity, bonds, or a basket of assets like an index fund. Unlike mutual funds, an ETF trades like a common stock on a stock exchange. ETFs experience price changes throughout the day as they are bought and sold. ETFs typically have higher daily liquidity and lower fees than mutual fund shares, making them an attractive alternative for individual investors.

Because it trades like a stock, an ETF does not have its net asset value (NAV) calculated once at the end of every day like a mutual fund does.

Just a few senteces and the definition,the rest you can find here : https://www.investopedia.com/terms/e/etf.asp