An ETF is a collection of stocks or bonds that may be purchased for one price. Unlike mutual funds, ETFs may be bought and sold during the entire trading day just like a stocks on an exchange. Many popular ETFs track well-known stock indexes like the S&P 500.
There are several differences between ETFs and mutual funds. Whereas mutual funds tend to have human mutual fund managers who actively trade stocks in and out of the fund based on which ones they predict will go up or down, the vast majority of ETFs are unmanaged by humans.