Investors often use major indices like the S&P 500, Nasdaq Composite, and Dow Jones Industrial Average as a way to understand how financial markets are performing. While this is a helpful place to start, the reality is that the stock market is made up of many thousands of companies, each affected by economic and market trends in unique ways. To understand these trends, stocks are often grouped into sectors, geography, and by other characteristics such as the size of the company or the style of investment. Each of these groups can play an important role in long-term portfolios.