US: Understanding market indices (S&P 500, Dow, Nasdaq)

When the news reports "the market," it usually means one of three US indices — baskets of stocks that gauge market performance.

The big three

• S&P 500: ~500 of the largest US companies, weighted by size. The best broad gauge of the US stock market and the benchmark most investors track.
• Dow Jones Industrial Average (the "Dow"): just 30 large companies, price-weighted (an older, narrower measure that still makes headlines).
• Nasdaq Composite / Nasdaq-100: heavily weighted toward technology and growth companies, so it's more volatile.

Why indices matter to you

• They're a benchmark: most active funds fail to beat the S&P 500 over the long run — a key argument for index investing.
• You can own them directly and cheaply through index funds and ETFs that track them, instantly diversifying across hundreds of companies.

A word on concentration

The S&P 500 is US-only and increasingly dominated by a handful of giant tech firms. Pairing it with international funds improves diversification.

Key takeaway

The S&P 500 is the go-to snapshot of the US market and a low-cost core holding via an index fund. Remember it's still one country — add global exposure for a truly diversified portfolio.