Indices are statistical measures that represent the performance of a group of assets, such as stocks or bonds. They are often used to track the overall health of a financial market or economy. For example, the S&P 500 index includes 500 of the largest publicly traded companies in the United States, providing a snapshot of the stock market's performance. Investors use indices to compare the performance of individual investments against a benchmark. This helps them make informed decisions about buying or selling assets. Common indices include the Dow Jones Industrial Average and the NASDAQ Composite, each representing different segments of the market.