ETFs trade like stocks on an exchange, and their price changes throughout the day as shares are bought and sold.
Most ETFs are considered passive investments because they are based on an index. Index-based ETFs are similar to those mutual funds known as "index funds," meaning they track market indexes to replicate the performance of a certain part of the market. For example, an ETF that tracks the S&P 500® index (SPX) is trying to mirror the performance of companies in the S&P 500. ETFs trade like stocks on an exchange, and their price changes throughout the day as shares are bought and sold.