When the news says “the market was up today,” it almost always means an index moved. An index is a single number that summarizes how a whole basket of stocks is doing.
A stock market index tracks the combined performance of a group of stocks, boiling them down to one number you can follow over time. Instead of checking hundreds of companies, you glance at the index and instantly know whether the market — or a slice of it — rose or fell.
| Index | What it tracks |
|---|---|
| S&P 500 | 500 of the largest U.S. companies — the broad market benchmark. |
| Dow Jones | 30 large, well-known “blue-chip” companies. |
| Nasdaq Composite | Thousands of stocks, heavily weighted toward tech. |
Not every stock counts equally. Most major indexes are market-cap weighted — bigger companies (by market cap) move the index more than small ones. The Dow is unusual: it's price-weighted, so a high-priced stock sways it more regardless of company size. Weighting explains why two indexes can disagree on the same day.
Indexes serve as benchmarks: a fund manager “beat the market” if they outperformed an index like the S&P 500. They're also investable — an index ETF lets you own the whole basket in one trade, which is the simplest path to broad diversification.
Not advice: educational content only. For official index and fund basics see investor.gov.
Connect the dots with what is an ETF, what is market cap, and the wider view in how the stock market works.
It is a single number that tracks the combined performance of a group of stocks, letting you see at a glance whether that part of the market rose or fell.
The S&P 500 (500 large U.S. companies), the Dow Jones Industrial Average (30 blue-chip companies), and the Nasdaq Composite (thousands of stocks, heavily weighted toward technology).
Most major indexes are market-cap weighted, so larger companies move them more. The Dow is price-weighted, meaning higher-priced stocks have more influence regardless of company size.
Not directly, but you can buy an index ETF or mutual fund that holds the same stocks in the same proportions, giving you the index's performance in a single investment.