Intro to ETFs & Indexes
What a market index measures, how ETFs let you buy a whole basket in one trade, and what to check before buying one.
What an index is
A stock market index is a scoreboard that tracks a group of stocks. The S&P 500 follows 500 large US companies chosen by a committee. The Nasdaq Composite covers the stocks listed on the Nasdaq exchange. The Dow Jones Industrial Average follows 30 large, well-known US companies. When you hear "the market was up today," people usually mean one of these indexes.
You cannot buy an index directly. It is just a calculation. But you can buy funds designed to track one.
How indexes are weighted
Indexes combine their members in different ways. The S&P 500 and Nasdaq Composite are market-cap weighted, so the biggest companies have the most influence. The Dow is price weighted, meaning higher-priced shares move it more regardless of company size. Some funds use equal weighting, giving every member the same slice.
| Weighting | How it works | Example |
|---|---|---|
| Market-cap weighted | Bigger companies count more | S&P 500, Nasdaq Composite |
| Price weighted | Higher share prices count more | Dow Jones Industrial Average |
| Equal weighted | Every company counts the same | Equal-weight index funds |
Three common ways to build an index.
ETFs: a basket in one ticker
An exchange-traded fund, or ETF, is a fund that holds a basket of investments and trades on an exchange like a single stock. Buy one share of an S&P 500 ETF and you indirectly own a small slice of every company in the index. ETFs can track broad markets, specific sectors, bonds, commodities, or themes.
Why ETFs are popular
- Instant diversification: one purchase can spread your money across hundreds of companies.
- Low cost: many broad index ETFs charge very small yearly fees.
- Easy to trade: they trade all day like stocks, with regular and limit orders.
- Transparent: most ETFs publish their holdings regularly.
The expense ratio
Every fund charges a yearly fee called the expense ratio, expressed as a percentage of your investment. An expense ratio of 0.10% costs $1 per year for every $1,000 invested. Because fees compound against you over time, choosing a lower-cost fund that tracks the same index can meaningfully improve long-term results.
A real index fund chart
Broad index funds still rise and fall. They smooth out single-company risk but not market-wide drops, as the chart below shows.
A real daily chart of a widely held S&P 500 ETF. Diversified, but still subject to market swings.
What to check before buying an ETF
Before buying any ETF, look at what it actually holds, what it costs, and how easy it is to trade. Two funds with similar names can hold very different things. Narrow or leveraged funds behave very differently from broad index funds and can be much riskier.
- Holdings: which index or strategy does it follow, and what are its biggest positions?
- Expense ratio: lower is better when two funds track the same thing.
- Liquidity: average volume and bid-ask spread.
- Special features: leveraged or inverse ETFs reset daily and are built for short-term trading, not long holding.
ETFs vs. mutual funds
ETFs and mutual funds both pool money into baskets of investments. The main practical difference is how they trade. ETFs trade throughout the day on an exchange at market prices, like a stock. Traditional mutual funds are bought and sold once a day at a price calculated after the close. Both can be low cost, and many index mutual funds and index ETFs track the same benchmarks.
| ETF | Mutual fund | |
|---|---|---|
| How it trades | All day on an exchange | Once a day at the closing value |
| Order types | Market, limit, stop | Buy/sell at next calculated price |
| Minimums | Usually one share (or a fraction) | Sometimes a set minimum |
General differences; specific funds vary.
How beginners often use them
A common starting point is a single broad-market ETF as a core holding, added to regularly. From there, some investors add a bond fund to reduce swings, or a sector ETF to express a view without betting on a single company. Traders also watch index ETFs to read the market’s overall direction before taking individual setups, which you will see throughout Chart School.
Costs you might not see
The expense ratio isn’t the only cost of owning an ETF. The bid-ask spread is another. Popular ETFs usually have spreads of a penny or two, but thinly traded ones can be much wider. There is also tracking difference: how closely the fund’s return matches its index after fees. For big, heavily traded index ETFs these costs are tiny. For niche funds, check the average daily volume and the typical spread before you buy.
Key takeaways
- An index is a calculated scoreboard for a group of stocks; you buy funds that track it.
- Cap-weighted indexes are driven by the biggest companies; the Dow is price weighted.
- An ETF holds a basket of investments and trades like a single stock.
- The expense ratio is the yearly fee — 0.10% is $1 per $1,000 per year.
- Check holdings, cost and liquidity; leveraged and inverse ETFs are short-term tools.