Securities that are similar to mutual funds in that they track an index, a commodity or a group of assets. But they trade like a stock on an exchange. Their prices also change through the day as they are bought and sold. They allow investors a convenient way to purchase a broad array of securities in a single transaction, and because they are not actively managed like a mutual fund, their expense ratios are low. They also can be bought through discount brokers that charge less than $10 a trade.