Each index measures a group of the stock market. Usually, the group is a country but it can be just an industry sector, like technology or industrial. Here are the most popular: Dow Jones (DJ 30), also known as Dow Jones Industrial Average, is made up of the 30 biggest industrial companies in the U.S. S&P 500, it’s the biggest index in the world. It represents 500 companies from the U.S. NASDAQ 100, measures the 100 major tech companies in the U.S. DAX 30, it’s the German index. It includes the 30 leading German companies. FTSE 100, groups the top 100 companies of the U.K. CAC 40, represents the best 40 French companies. NIKKEI 225, it’s the index from Japan. It contains the 225 biggest companies in the country.