A stock market index is a measurement of a specific segment of the stock market — calculated from the prices of selected stocks. When you hear “the market was up today,” someone is almost always referring to a specific index. Understanding the major indices helps you understand what’s actually being talked about.
The S&P 500
The S&P 500 tracks 500 of the largest publicly traded US companies. It’s market-cap weighted — bigger companies have more influence on the index. When most people say “the market,” they mean the S&P 500. It’s considered the best single measure of the US large-cap equity market and has returned approximately 10% annually on average over the past century.
The Dow Jones Industrial Average (DJIA)
The Dow tracks just 30 large US companies — selected to represent the broader economy. It’s price-weighted (higher-priced stocks have more influence, regardless of company size). The Dow gets a lot of media attention but is considered a less comprehensive measure of the market than the S&P 500.
The Nasdaq Composite
The Nasdaq tracks over 3,000 companies listed on the Nasdaq exchange — heavily weighted toward technology companies. When you hear about “tech stocks” having a good or bad day, the Nasdaq is usually the reference point. More volatile than the S&P 500 due to the concentration in growth-oriented tech companies.
Why indices matter for investors
Index funds track these indices — when you invest in an S&P 500 index fund, you’re buying tiny pieces of all 500 companies in the index. Your investment rises and falls with the index. This is why “the market is up” is directly relevant to your portfolio if you hold index funds — which most long-term investors do.