Indexes and ETFs

An index tracks a group of stocks to show how that part of the market is doing. When people say "the market was up today," they usually mean an index like the S&P 500.

• S&P 500: 500 large U.S. companies; the most watched measure of the U.S. market.
• Nasdaq-100: 100 large companies on the Nasdaq exchange, heavy in technology.
• Dow Jones Industrial Average: 30 large, well-known companies.
• Russell 2000: 2,000 smaller companies.