Exchange Traded Fund (ETF): It’s an investment fund that explicitly tracks an underlying “asset” like the price of oil or the value of US stocks (as determined by the S&P 500 or another large index). Investors can buy and sell it just like a normal stock – which makes it easy to invest in. Typically, ETFs have low fees as they simply track the value of something else, i.e. it is not trying to pick winning stocks. Robo-advisors and other firms often use ETFs to create low-cost investment portfolios for their clients.