Market indices are statistical measures that track the performance of a specific group of stocks or other financial assets. They help investors gauge the overall health of the stock market or a particular sector. Common examples include the S&P 500, which represents 500 of the largest U.S. companies, and the Dow Jones Industrial Average, which includes 30 major U.S. corporations. These indices are calculated using various methods, such as price-weighting or market capitalization. By monitoring market indices, investors can make informed decisions about buying or selling assets, as these indices reflect market trends and investor sentiment.