For most of the twentieth century, investing meant picking stocks or paying a professional to pick them for you. Index funds and exchange-traded funds, known as ETFs, changed that. Today they hold trillions of dollars and are the default choice for millions of retirement savers. Both rest on the same idea: instead of trying to beat the market, simply own it.

What an Index Fund Is

An index is a list of securities that represents a market or a slice of one. The S&P 500 tracks 500 of the largest U.S. companies, while the Nasdaq-100 focuses on large non-financial firms listed on the Nasdaq exchange. An index fund is a fund that buys the securities in an index, in the same proportions, and holds them. Because no one is researching companies or making trading calls, the approach is called passive investing. The first index fund available to ordinary investors was launched by Vanguard in 1976. Critics at the time dismissed it as settling for average. That average turned out to be hard to beat.

How ETFs Differ

An ETF usually tracks an index too, but it is packaged differently. A traditional index mutual fund is bought and sold once a day, directly with the fund company, at a price calculated after the market closes. An ETF trades on a stock exchange throughout the day, just like a share of a company. The first U.S. ETF, which tracks the S&P 500, began trading in 1993. That structure gives ETFs some practical advantages. Investors can buy a single share, there is often no minimum investment, and the way ETFs create and redeem shares tends to make them more tax-efficient. Mutual funds, on the other hand, make automatic recurring investments simple, which is why they remain common in workplace retirement plans.

Why Low Costs Matter

The main selling point of both products is cost. Every fund charges an expense ratio, a yearly fee taken as a percentage of the money invested. Broad index funds and ETFs often charge less than 0.10%, while actively managed funds commonly charge several times that. The gap sounds small, but fees compound just as returns do. Over several decades, a difference of one percentage point a year can consume a large share of an investor’s final balance. Cost is also the main reason passive funds tend to win. Studies that compare active managers with their benchmarks have repeatedly found that a large majority fall behind over periods of ten years or more, largely because of fees.

The Limits of Passive Investing

Index investing is not risk-free. A fund that owns the whole market falls when the market falls, and no manager will step in to limit the damage. Most major indexes are also weighted by company size, so a handful of very large firms can make up a big portion of the fund. An investor in an S&P 500 fund owns far more of the largest technology companies than of the smallest members of the index. Not every ETF is a simple, diversified product either. Some track narrow themes, use borrowed money to magnify daily moves, or hold a single commodity. These can behave very differently from a broad market fund.

Conclusion

Index funds and ETFs give investors instant diversification at very low cost, and the evidence suggests that is enough to outperform most professionals over time. The choice between the two mostly comes down to convenience: ETFs offer flexibility and tax efficiency, while index mutual funds suit automatic investing. What matters more than the wrapper is what is inside it. A broad, cheap fund held for many years remains one of the most reliable tools available to an ordinary investor.